libgomp nvptx plugin: Split 'PLUGIN_NVPTX_DYNAMIC' into 'PLUGIN_NVPTX_INCLUDE_SYSTEM_...

libgomp nvptx plugin: Split 'PLUGIN_NVPTX_DYNAMIC' into 'PLUGIN_NVPTX_INCLUDE_SYSTEM_CUDA_H' and 'PLUGIN_NVPTX_LINK_LIBCUDA'

Including the GCC-shipped 'include/cuda/cuda.h' vs. system <cuda.h> and
'dlopen'ing the CUDA Driver library vs. linking it are separate concerns.

	libgomp/
	* plugin/Makefrag.am: Handle 'PLUGIN_NVPTX_DYNAMIC'.
	* plugin/configfrag.ac (PLUGIN_NVPTX_DYNAMIC): Change
	'AC_DEFINE_UNQUOTED' into 'AM_CONDITIONAL'.
	* plugin/plugin-nvptx.c: Split 'PLUGIN_NVPTX_DYNAMIC' into
	'PLUGIN_NVPTX_INCLUDE_SYSTEM_CUDA_H' and
	'PLUGIN_NVPTX_LINK_LIBCUDA'.
	* Makefile.in: Regenerate.
	* config.h.in: Likewise.
	* configure: Likewise.