configure: Account CXXFLAGS in gcc-plugin.m4.

We now use a C++ compiler so that we need to process
CXXFLAGS as well as CFLAGS in the gcc-plugin config
fragment.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

config/ChangeLog:

	* gcc-plugin.m4: Save and process CXXFLAGS.

gcc/ChangeLog:

	* configure: Regenerate.

libcc1/ChangeLog:

	* configure: Regenerate.