configure: add --enable-werror
configure: add --enable-werror

Provide a sane means of adding the -Werror compiler flag.

Currently, some of our jenkins.sh add -Werror by passing 'CFLAGS="-Werror"',
but that actually *overwrites* all the other CFLAGS we might want to have set.

Maintain these exceptions from -Werror:
a) deprecation (allow upstream to mark deprecation without breaking builds);
b) "#warning" pragmas (allow to remind ourselves of errors without breaking
   builds)

As a last configure step before generating the output files, print the complete
CFLAGS and CPPFLAGS by means of AC_MSG_RESULT.

Change-Id: I20d5f18cf05c9e277d914ada259a95187b73fe39

osmocom/libosmo-abis,osmocom/libosmo-abis,osmocom/libosmo-abis