Change GnuCash CMakeLists.txt for C++17

gistfile1.txt

--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -532,18 +532,22 @@ endif()
 add_definitions(-D_GNU_SOURCE)

 # Also, set the C++ version to c++11
-set(CMAKE_CXX_FLAGS "-std=gnu++11 ${CMAKE_CXX_FLAGS}")  # FIXME: should be -std=c++11
-
+set(CMAKE_CXX_STANDARD 17)
+set(CMAKE_CXX_STANDARD_REQUIRED ON)
+set(CMAKE_CXX_EXTENSIONS OFF)
+set(CMAKE_C_STANDARD 11)
+set(CMAKE_C_STANDARD_REQUIRED ON)
+set(CMAKE_C_EXTENSIONS ON)

 if (UNIX)
   set( CMAKE_C_FLAGS "-Werror -Wdeclaration-after-statement -Wno-pointer-sign -Wall -Wmissing-prototypes -Wmissing-declarations -Wno-unused ${CMAKE_C_FLAGS}")
-  set( CMAKE_C_FLAGS "-Wno-error=deprecated-declarations -std=gnu11 -Wno-error=parentheses ${CMAKE_C_FLAGS}")
+  set( CMAKE_C_FLAGS "-Wno-error=deprecated-declarations -Wno-error=parentheses ${CMAKE_C_FLAGS}")
   set( CMAKE_CXX_FLAGS "-Werror -Wall -Wmissing-declarations -Wno-unused -Wno-error=parentheses ${CMAKE_CXX_FLAGS}")
   set( CMAKE_CXX_FLAGS "-Wno-error=deprecated-declarations ${REGISTER_CXXFLAG} ${CMAKE_CXX_FLAGS}")
   set( CMAKE_C_FLAGS_RELEASE "-O3 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=2 ${CMAKE_C_FLAGS}")
 endif (UNIX)
 if (MINGW)
-  set( CMAKE_C_FLAGS "-Werror -Wdeclaration-after-statement -Wno-pointer-sign -Wall -Wunused -Wmissing-prototypes -Wmissing-declarations  -Wno-unused -Wno-error=deprecated-declarations -std=gnu11 ${CMAKE_C_FLAGS}")
+  set( CMAKE_C_FLAGS "-Werror -Wdeclaration-after-statement -Wno-pointer-sign -Wall -Wunused -Wmissing-prototypes -Wmissing-declarations  -Wno-unused -Wno-error=deprecated-declarations ${CMAKE_C_FLAGS}")
   set( CMAKE_CXX_FLAGS "-DWINVER=0x0500 -D_EMULATE_GLIBC=0 ${CMAKE_CXX_FLAGS}") # Workaround for bug in gtest on mingw, see https://github.com/google/googletest/issues/893 and https://github.com/google/googletest/issues/920
 endif (MINGW)