FindQuadmath: add -fext-numeric-literals if the compiler supports it

this is required for GCC >= 4.8 to support the 'Q' suffix for floating
point literals (which are used in the quadmath.h header)

In order to prevent compilers which eat the flag but do not support it
from complaining constantly, it is tested for with "-Werror" enabled...
add check-commits target
synchronize build system with opm-core

for some reason, parts of d1020930. must have been a screw-up of the
opm-material maintainer...