Extract detecting CC/CXX into helper script so we can use it from mono.proj

[akoeplinger]

Also fixed a small typo in the detection script, see first commit.

Closes #31771

[am11]

Sorry about that typo, I happened to stumble upon that condition today on different platform and fixed in #31865 😄 (will revert from there).
+1 for reusing the code.

[davidwrighton]

@akoeplinger This works. It fixes the issue I hit nicely. I'm going to close my PR as this supersedes it.

[am11]

@akoeplinger, would it be possible to let eng/build.sh determine which compiler to use? i.e. the hardcoded clang to be replaced by $(Compiler) defined here:

runtime/eng/build.sh

Lines 275 to 292 in 2ef008e

	-clang*)
	arguments="$arguments /p:Compiler=$opt"
	shift 1
	;;
	-cmakeargs)
	if [ -z ${2+x} ]; then
	echo "No cmake args supplied." 1>&2
	exit 1
	fi
	cmakeargs="${cmakeargs} ${opt} $2"
	shift 2
	;;
	-gcc*)
	arguments="$arguments /p:Compiler=$opt"
	shift 1
	;;

Doing that will help systems which require gcc build (runtime/build.sh -gcc ...).

[akoeplinger]

@am11 sure, that would be great!