Fix warning due to usage of GNU extension `##__VA_ARGS__`

[thilinarmtb]

Currently, the only warning I see when building libnomp is the following:

In file included from /Users/thili/Repos/nomp-org/libnomp/src/nomp.c:1:
In file included from /Users/thili/Repos/nomp-org/libnomp/include/nomp-impl.h:26:
/Users/thili/Repos/nomp-org/libnomp/include/nomp-log.h:51:52: warning: token pasting of ',' and __VA_ARGS__ is a GNU extension [-Wgnu-zero-variadic-macro-arguments]
  nomp_log_(desc, logno, type, __FILE__, __LINE__, ##__VA_ARGS__)

This is due to the usage of ##__VA_ARGS__ which is a GNU extension (although widely supported by different compilers).

Once this is fixed, we should treat warnings as errors in CMakeLists.txt:

set(CMAKE_COMPILE_WARNING_AS_ERROR ON)