Preprocessor token pasting in GCC’s _Pragma operator

After much trial and error, it turns out that the simplest solution works:

#ifdef _OPENMP
#   define LIB_PRAGMA_OMP(x)  DO_PRAGMA(omp x)
#   define DO_PRAGMA(x) _Pragma ( #x )
#else
#   define LIB_PRAGMA_OMP(x)
#endif

With -DOPENMP, I get:

# 12 "test_op.cpp"
#pragma omp parallel
# 12 "test_op.cpp"

And without it, nothing.