support: build ZIG0 only; build ZIG with a pre-built ZIG0

cmake.diff

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 27ed0ac73..9cf132a52 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -91,6 +91,9 @@ set(ZIG_SINGLE_THREADED off CACHE BOOL "limit the zig compiler to use only 1 thr
 set(ZIG_OMIT_STAGE2 off CACHE BOOL "omit the stage2 backend from stage1")
 set(ZIG_ENABLE_LOGGING off CACHE BOOL "enable logging")
 
+set(ZIG0_ONLY off CACHE BOOL "Build only zig0")
+set(ZIG0_EXECUTABLE "" CACHE STRING "Path to already-built zig0 binary")
+
 if("${ZIG_TARGET_TRIPLE}" STREQUAL "native")
     set(ZIG_USE_LLVM_CONFIG ON CACHE BOOL "use llvm-config to find LLVM libraries")
 else()
@@ -746,7 +749,7 @@ if(MSVC OR MINGW)
     target_link_libraries(zigstage1 LINK_PUBLIC version)
 endif()
 
-if("${ZIG_EXECUTABLE}" STREQUAL "")
+if("${ZIG_EXECUTABLE}" STREQUAL "" AND "${ZIG0_EXECUTABLE}" STREQUAL "")
   add_executable(zig0 ${ZIG0_SOURCES})
   set_target_properties(zig0 PROPERTIES
       COMPILE_FLAGS ${EXE_CFLAGS}
@@ -787,70 +790,78 @@ set(BUILD_ZIG1_ARGS
     --pkg-end
 )
 
-if("${ZIG_EXECUTABLE}" STREQUAL "")
-  add_custom_command(
-      OUTPUT "${ZIG1_OBJECT}"
-      COMMAND zig0 ${BUILD_ZIG1_ARGS}
-      DEPENDS zig0 "${ZIG_STAGE2_SOURCES}"
-      COMMENT STATUS "Building self-hosted component ${ZIG1_OBJECT}"
-      WORKING_DIRECTORY "${CMAKE_SOURCE_DIR}"
-  )
-  set(ZIG_EXECUTABLE "${zig_BINARY_DIR}/zig")
-  if (WIN32)
-    set(ZIG_EXECUTABLE "${ZIG_EXECUTABLE}.exe")
-  endif()
-else()
-  add_custom_command(
-      OUTPUT "${ZIG1_OBJECT}"
-      COMMAND "${ZIG_EXECUTABLE}" "build-obj" ${BUILD_ZIG1_ARGS}
-      DEPENDS ${ZIG_STAGE2_SOURCES}
-      COMMENT STATUS "Building self-hosted component ${ZIG1_OBJECT}"
-      WORKING_DIRECTORY "${CMAKE_SOURCE_DIR}"
-  )
-endif()
+if(NOT ZIG0_ONLY)
+    if("${ZIG0_EXECUTABLE}" STREQUAL "")
+        set(ZIG0 "zig0")
+    else()
+        set(ZIG0 "${ZIG0_EXECUTABLE}")
+    endif()
 
-# cmake won't let us configure an executable without C sources.
-add_executable(zig "${CMAKE_SOURCE_DIR}/src/stage1/empty.cpp" "${ZIG1_OBJECT}")
+    if("${ZIG_EXECUTABLE}" STREQUAL "")
+      add_custom_command(
+          OUTPUT "${ZIG1_OBJECT}"
+          COMMAND ${ZIG0} ${BUILD_ZIG1_ARGS}
+          DEPENDS ${ZIG0} "${ZIG_STAGE2_SOURCES}"
+          COMMENT STATUS "Building self-hosted component ${ZIG1_OBJECT}"
+          WORKING_DIRECTORY "${CMAKE_SOURCE_DIR}"
+      )
+      set(ZIG_EXECUTABLE "${zig_BINARY_DIR}/zig")
+      if (WIN32)
+        set(ZIG_EXECUTABLE "${ZIG_EXECUTABLE}.exe")
+      endif()
+    else()
+      add_custom_command(
+          OUTPUT "${ZIG1_OBJECT}"
+          COMMAND "${ZIG_EXECUTABLE}" "build-obj" ${BUILD_ZIG1_ARGS}
+          DEPENDS ${ZIG_STAGE2_SOURCES}
+          COMMENT STATUS "Building self-hosted component ${ZIG1_OBJECT}"
+          WORKING_DIRECTORY "${CMAKE_SOURCE_DIR}"
+      )
+    endif()
 
-set_target_properties(zig PROPERTIES
-    COMPILE_FLAGS ${EXE_CFLAGS}
-    LINK_FLAGS ${EXE_LDFLAGS}
-)
-target_link_libraries(zig zigstage1)
-if(MSVC)
-  target_link_libraries(zig ntdll.lib)
-elseif(MINGW)
-  target_link_libraries(zig ntdll)
-endif()
+    # cmake won't let us configure an executable without C sources.
+    add_executable(zig "${CMAKE_SOURCE_DIR}/src/stage1/empty.cpp" "${ZIG1_OBJECT}")
 
-install(TARGETS zig DESTINATION bin)
+    set_target_properties(zig PROPERTIES
+        COMPILE_FLAGS ${EXE_CFLAGS}
+        LINK_FLAGS ${EXE_LDFLAGS}
+    )
+    target_link_libraries(zig zigstage1)
+    if(MSVC)
+      target_link_libraries(zig ntdll.lib)
+    elseif(MINGW)
+      target_link_libraries(zig ntdll)
+    endif()
 
-set(ZIG_SKIP_INSTALL_LIB_FILES off CACHE BOOL
-    "Disable copying lib/ files to install prefix during the build phase")
+    install(TARGETS zig DESTINATION bin)
 
-if(NOT ZIG_SKIP_INSTALL_LIB_FILES)
-    set(ZIG_INSTALL_ARGS "build"
-        --override-lib-dir "${CMAKE_SOURCE_DIR}/lib"
-        "-Dlib-files-only"
-        --prefix "${CMAKE_INSTALL_PREFIX}"
-        "-Dconfig_h=${ZIG_CONFIG_H_OUT}"
-        install
-    )
+    set(ZIG_SKIP_INSTALL_LIB_FILES off CACHE BOOL
+        "Disable copying lib/ files to install prefix during the build phase")
 
-    # CODE has no effect with Visual Studio build system generator, therefore
-    # when using Visual Studio build system generator we resort to running
-    # `zig build install` during the build phase.
-    if(MSVC)
-        add_custom_target(zig_install_lib_files ALL
-            COMMAND zig ${ZIG_INSTALL_ARGS}
-            DEPENDS zig
-            WORKING_DIRECTORY "${CMAKE_SOURCE_DIR}"
+    if(NOT ZIG_SKIP_INSTALL_LIB_FILES)
+        set(ZIG_INSTALL_ARGS "build"
+            --override-lib-dir "${CMAKE_SOURCE_DIR}/lib"
+            "-Dlib-files-only"
+            --prefix "${CMAKE_INSTALL_PREFIX}"
+            "-Dconfig_h=${ZIG_CONFIG_H_OUT}"
+            install
         )
-    else()
-        get_target_property(zig_BINARY_DIR zig BINARY_DIR)
-        install(CODE "set(zig_EXE \"${ZIG_EXECUTABLE}\")")
-        install(CODE "set(ZIG_INSTALL_ARGS \"${ZIG_INSTALL_ARGS}\")")
-        install(CODE "set(CMAKE_SOURCE_DIR \"${CMAKE_SOURCE_DIR}\")")
-        install(SCRIPT ${CMAKE_CURRENT_SOURCE_DIR}/cmake/install.cmake)
+
+        # CODE has no effect with Visual Studio build system generator, therefore
+        # when using Visual Studio build system generator we resort to running
+        # `zig build install` during the build phase.
+        if(MSVC)
+            add_custom_target(zig_install_lib_files ALL
+                COMMAND zig ${ZIG_INSTALL_ARGS}
+                DEPENDS zig
+                WORKING_DIRECTORY "${CMAKE_SOURCE_DIR}"
+            )
+        else()
+            get_target_property(zig_BINARY_DIR zig BINARY_DIR)
+            install(CODE "set(zig_EXE \"${ZIG_EXECUTABLE}\")")
+            install(CODE "set(ZIG_INSTALL_ARGS \"${ZIG_INSTALL_ARGS}\")")
+            install(CODE "set(CMAKE_SOURCE_DIR \"${CMAKE_SOURCE_DIR}\")")
+            install(SCRIPT ${CMAKE_CURRENT_SOURCE_DIR}/cmake/install.cmake)
+        endif()
     endif()
 endif()