devel/dmlc-core: fix build on GCC architectures
Use USES=compiler:openmp instead of hard dependency on ports LLVM. Use new GCC when using base GCC.
Remove -D_POSIX_SOURCE -D_POSIX_C_SOURCE=200809L, this causes some errno.h macros not being visible.
PR: 242234
Approved by: yuri (maintainer), linimon (mentor)
Differential Revision: https://reviews.freebsd.org/D22580