Previously we checked for this flag by parsing the version number of GCC
out of 'gcc --version', but this is not reliable because the format can
vary greatly.  Now we run 'gcc -v --help' and look for '-isysroot' in
the list of options.

We also now store the result on a per-language basis in the per-compiler
info file "CMake<LANG>Compiler.cmake".  This is necessary to make it
accessible from try-compile projects so that they generate correctly.