Previously subprojects inherited languages already added by main
project, or any previous subproject. This change to have a list of
compilers per interpreters, which means that if a subproject does not
add 'c' language it won't be able to compile .c files any more, even if
main project added the 'c' language.
This delays processing list of compilers until the interpreter adds the
BuildTarget into its list of targets. That way the interpreter can add
missing languages instead of duplicating that logic into BuildTarget for
the cython case.
Xavier Claessens