nvhpc: lots of warnings from 'pragma GCC system_header'

[cwsmith]

On ALCF Polaris using the nvhpc 23.9.0 compilers, this macro:

omega_h/src/Omega_h_macros.h

Lines 16 to 22 in 0dcdfee

	#if defined(_MSC_VER)
	#define OMEGA_H_SYSTEM_HEADER
	#elif defined(__clang__)
	#define OMEGA_H_SYSTEM_HEADER OMEGA_H_PRAGMA(clang system_header)
	#elif defined(__GNUC__)
	#define OMEGA_H_SYSTEM_HEADER OMEGA_H_PRAGMA(GCC system_header)
	#endif

results in pragma GCC system_header being added to a handful of header files. The compiler/pre processor outputs the following warning:

omega_h/src/Omega_h_mpi.h(8): warning #2320-D: #pragma GCC system_header cannot be used in the primary source file
  _Pragma("GCC system_header")