Hadrian does not propagate CONF_CC_OPTS_STAGE2 to in-tree GMP

When GHC is configured with --with-intree-gmp, hadrian invokes the GMP configure script, but it doesn't propagate the CONF_CC_OPTS_STAGE2 flags to the CFLAGS environment variable when running GMP configure. This is troublesome when I'd like to customize how the in-tree GMP is built, especially when debugging and I'd like to add -g (right, debug_info doesn't affect in-tree GMP as well and that's also a bug?)