Pass the full CFLAGS to cpp for MKlib_gen.sh.

[jhb]

Summary

GCC's cpp was exiting when it failed to find requested includes
(<ncurses_cfg.h> and <ncurses_defs.h>). clang-cpp just silently
ignored the missing header files (thus not honoring any macros
defined in those).

Test Plan

• build worlds with amd64-gcc and aarch64-gcc which failed to link lldb since libc_gen.c was empty and lldb links against box()

[jhb]

This is a regression introduced earlier this year in an ncurses upgrade when this was switched to using CPP + CPPFLAGS (which is always empty) instead of CC + CFLAGS (which the other generated files still use?). The fact that clang-cpp silently fails and ignores errors seems like a bug in clang-cpp to me.

[kevans]

LGTM and explains what I had observed when looking at this a couple weeks ago.