configure: detect a clang build

which is triggered by:

./configure ....  CC=clang CXX=clang++
Makefile: adapt CFLAGS conditionally on clang
clang/fxes: webtalk

clang is pretty good at dignosing int overflow issues
clang: proper casts in emcmodule.cc
tp/blendmath: make negate a static inline to keep clang 3.5 happy

without this, it would result in a undefined symbol 'negate' in motmod.so
if built with clang 3.5

looks like a clang bug