Don't define SLURM_CONFIG_FILE and SLURM_PLUGIN_PATH in CPPFLAGS. This

requires that all source files be rebuilt if sysconfdir or pkglibdir
changes. 

Instead, src/common/Makefile now generates global_defaults.c, which 
contains global symbols "default_plugin_path" and "default_slurm_config_file"
which means that only this source file needs to be rebuilt, and
libcommon relinked when sysconfdir or pkglibdir are redefined.