Enable strict flags in a makefile for specific files

example.mk

# Enable very strict flags for some files to ensure compatibility with
# all compilers (enables C99 strict compliance mode and more warnings).
STRICTFLAGS := \
	-D_XOPEN_SOURCE --std=c99 -pedantic-errors -W -Wall -Wextra \
	-Wno-variadic-macros -Wno-overlength-strings -fdiagnostics-show-option
STRICTFLAGS_FILES := \
	file_a.c \
	file_b.c

# Use '=' not ':=' to create a makefile macro
STRICTFLAGS_MACRO = $(if $(findstring $(notdir $<),$(STRICTFLAGS_FILES)),$(STRICTFLAGS))