The current way we're handling FLAGS (CFLAGS/CXXFLAGS, LDFLAGS, etc) is laden with technical debt. It's a sorry mess, hard to do anything constructive about.

This is the first step towards a better solution. The main goal is to have a set of more specific flags, more like how the SYSROOT flags work today, that can be individually included, instead of a huge CFLAGS_JDKLIB.

To make such a change manageable, this first step will (almost) only refactor the autoconf code, but will output the same flags in the spec.gmk.