configure.ac: Clear the implicit default C and C++ flags.

They were being added by Autoconf following our own defaults, leading
to the erroneous -g -O3 -g -O2 options being passed to GCC.  This is
documented in the C Compiler and C++ Compiler sections of the Autoconf
manual.

* configure.ac: Clear the default CFLAGS and CXXFLAGS variables.

Reported-by: Adrien Beraud <adrien.beraud@savoirfairelinux.com>
Change-Id: Id5e24cb2544ac4492b229746ad7ea1dee7cbb963