gistfile1.txt

/home/gk/src/master/proto.strap/usr/gcc/7/bin/gcc -dumpspecs>specs
...

*startfile_arch:
%{std=c1x|std=gnu1x:values-Xc.o%s values-xpg6.o%s;                             std=c11|std=gnu11:values-Xc.o%s values-xpg6.o%s;                                std=c99|std=gnu99:values-Xc.o%s values-xpg6.o%s;                                std=c9x|std=gnu9x:values-Xc.o%s values-xpg6.o%s;                                std=c++0x|std=gnu++0x:values-Xc.o%s;                            std=c++11|std=gnu++11:values-Xc.o%s;                            std=c++03|std=gnu++03:values-Xc.o%s;                            :values-Xa.o%s}
...

f905ddb5-9113-c52a-d3ce-d0db83321d72:lib $ echo '#include <sys/feature_tests.h>' | /home/gk/src/master/proto.strap/usr/gcc/7/bin/gcc -std=c99 -dM -E - >a.h
f905ddb5-9113-c52a-d3ce-d0db83321d72:lib $ grep SOURCE a.h
f905ddb5-9113-c52a-d3ce-d0db83321d72:lib $ grep XOPEN a.h
#define _XOPEN_VERSION 3
f905ddb5-9113-c52a-d3ce-d0db83321d72:lib $ grep POSIX a.h