Make available C_DIALECT_OPT_XXX variables for selecting C dialect.

If known, Compiler.cmake sets the four variables C_DIALECT_OPT_C89,
C_DIALECT_OPT_C89EXT, C_DIALECT_OPT_C99 and C_DIALECT_OPT_C99EXT,
corresponding to the compiler options to set the corresponding
C dialects: C89/C90, C89/90 with compiler extensions, C99, and
C99 with compiler extensions.

This allows projects to specify C dialect on a per target or project
basis.