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.
