configure: Allow user to specify compiler to be used.

configure: Allow user to specify compiler to be used.

The environment variables CC and CXX can be set at configure time to
specify what compiler to use. This compiler will be used for any
configure-time compilation, and will also be recorded in
Makefile.config to be used during the actual build.

The compiler to be used can still be overridden at build time by using
a make variable such as:

	make CC=gcc