Create FortranCInterface_VERIFY function

This function builds a simple test project using a combination of
Fortran and C (and optionally C++) to verify that the compilers are
compatible.  The idea is to help projects report very early to users
that the compilers specified cannot mix languages.

Modules/FortranCInterface/Detect.cmake

@@ -17,6 +17,10 @@ else()
   return()
 endif()
 
+# Invalidate verification results.
+unset(FortranCInterface_VERIFIED_C CACHE)
+unset(FortranCInterface_VERIFIED_CXX CACHE)
+
 set(_result)
 
 # Build a sample project which reports symbols.