libcc1: share basic context code

Both plugins in libcc1 share a fair amount of boilerplate.  They both
share error-emission code, context management code, and tree GC code.
This patch unifies these two bodies of code, avoiding needless
duplication.

libcc1

	* libcc1plugin.cc: Move code to context.cc.
	* libcp1plugin.cc: Move code to context.cc.
	* context.hh: New file.
	* context.cc: New file.
	* Makefile.in: Rebuild.
	* Makefile.am (AM_CPPFLAGS): Add more gcc flags.
	(CPPFLAGS_FOR_C, CPPFLAGS_FOR_CXX): Update.
	(libcc1plugin_la_SOURCES): Add context.hh, context.cc.
	(libcp1plugin_la_SOURCES): Likewise.