Combine *_EXTRA_DIST into EXTRA_DIST in Makefile.am
automake only recognizes EXTRA_DIST - but it looked like it worked since the *.pc.in were included as they're also listed in configure.ac, unless you tried building from the tarballs with --enable-legacy, and then it broke.
Also adds --enable-legacy to the flags that "make distcheck" uses to configure, to catch problems like this.
Fixes: #12 (closed)
The tarball created by "make dist" after this change contains all the files from before, plus these:
xorgproto-2018.4/autogen.sh
xorgproto-2018.4/man/Xprint.man
xorgproto-2018.4/man/Xprint.sgml
xorgproto-2018.4/specs/xp_proto_cov.mif
xorgproto-2018.4/specs/xp_proto.book
xorgproto-2018.4/specs/xp_proto.mif
xorgproto-2018.4/specs/xp_proto.ps
xorgproto-2018.4/specs/xp_protoIX.doc
xorgproto-2018.4/specs/xp_protoTOC.doc