configure: Support the capturing of CFLAGS and CXXFLAGS at configure time.

These variables can now be set via configure time via environment
variables like so:

	CFLAGS=-g ./configure

and subsequent builds will remember these values. The values can
still be overridden at compile time by passing make variables:

      make CFLAGS=-O2

The CXXFLAGS variable is optional. If unset at either configure
time or at compile time, it will inherit its value from the
CFLAGS variable. (Though if explicitly set at configure time
it must be explicitly overriden at compile time---just overriding
CFLAGS will not override CXXFLAGS as well.)