litecross: only build c,c++ frontends by default

others are untested and some are likely broken. add this config option
before including config.mak rather than after so that the user can
override it in config.mak if desired.

litecross/Makefile

@@ -5,7 +5,7 @@ BINUTILS_SRCDIR = BINUTILS_SRCDIR_not_set
 GCC_SRCDIR = GCC_SRCDIR_not_set
 MUSL_SRCDIR = MUSL_SRCDIR_not_set
 
-COMMON_CONFIG = 
+COMMON_CONFIG = --enable-languages=c,c++
 GCC_CONFIG = $(COMMON_CONFIG) 
 TOOLCHAIN_CONFIG = $(GCC_CONFIG)