When testing CMAKE_<LANG>_COMPILER_ID values, do not explicitly
dereference or quote the variable. We want if() to auto-dereference the
variable and not its value. Also replace MATCHES with STREQUAL where
equivalent.
The test overrides the CMAKE_C_FLAGS and CMAKE_Fortran_FLAGS to test
passing a specific flag to the compiler wrapper scripts. We fix it to
honor any outside flags needed for the real compiler.
The commit "FortranCInterface: Honor language flags in checks" taught
the FortranCInterface module to pass C and Fortran flags into its
detection and verification checks. We improve on the change to allow
the '=' character in the language flags. This requires passing the
cache entry type with the -D options.
We create test FortranC.Flags to try passing per-language flags from a
project into its FortranCInterface detect/verify checks. We wrap the
compilers with scripts that enforce presence of expected flags.
