Add ldc and clang as fallback compilers

When doing this to get a compiler:
cc = c_get_default_compiler()
dc = d_get_default_compiler()

On start it should look for supported compilers and return one:
cc = gcc or clang
dc = dmd or ldc or gdc

Do this:
1. In config the rscript needs to say require_c()
2. Raise will then look for the c compilers and requite that at least one is found.
3. Then when the user calls c_get_default_compiler() it will return the best default
4. Calling any other c_ methods will error if the c compiler and linker has not been set