build: Edit clang condition to utilize cpp version instead of OS version

Rather than detecting if the host is using Darwin, the cpp version script now checks if it's version report includes clang, at which point it will replace cpp with clang (in preprocessor mode). This is slightly less intrusive then the previous iteration as the regular flow of the script is maintained for most operating systems and `CPP` is only changed when needed.

Signed-off-by: Alan Everett <[email protected]>

Makefile

@@ -18,14 +18,13 @@ LD=$(CROSS_PREFIX)ld
 OBJCOPY=$(CROSS_PREFIX)objcopy
 OBJDUMP=$(CROSS_PREFIX)objdump
 STRIP=$(CROSS_PREFIX)strip
+CPP=cpp
 PYTHON=python3
 
-UNAME_S := $(shell uname -s)
-ifeq ($(UNAME_S),Darwin)
-# cpp on MacOS forces a specific input directory, so directly use clang instead
+CPP_VERSION := $(shell $(CPP) --version)
+ifeq ($(findstring clang,$(CPP_VERSION)),clang)
+# The clang version of cpp forces a specific input directory, so directly use clang instead
 CPP=clang -E
-else
-CPP=cpp
 endif
 
 # Source files