only set -fext-numeric-literals for c++ sources

[akva2]

get rid of warning

[288/821] Building C object CMakeFiles/opmsimulators.dir/opm/simulators/linalg/mixed/bsr.c.o
cc1: warning: command-line option '-fext-numeric-literals' is valid for C++/ObjC++ but not for C

[akva2]

jenkins build this please

[blattms]

Thanks a lot.
I recall a discussion in DUNE that this option is not needed if the standard gnu++17 is used. Never checked that myself and cannot find a reference 😅 Might not hold for OPM anyway