Add --scm option for %autosetup SCM selection

Allow selecting %autosetup SCM backend from cli in case it's not
hardwired in the spec, handy for eg rediffing patches.

doc/rpmbuild.8

@@ -111,6 +111,13 @@ will be run after a chroot(2) to
 .TP
 \fB-D, --define='\fIMACRO EXPR\fB'\fR
 Defines \fIMACRO\fR with value \fIEXPR\fR.
+.TP
+\fB--scm=\fISCM\fR
+Select the \fISCM\fR to use with %autosetup, if one is not set in the spec file.
+Note that not all values for \fISCM\fR, e.g., \fBpatch\fR (the default) and
+\fBgendiff\fR, \fBgit\fR, or \fBquilt\fR work interchangeably with all
+other patches and options stated in the %autosetup line, especially
+option \fB-p\fIN\fR.
 .SS "BUILD OPTIONS"
 .PP
 The general form of an rpm build command is 

rpmpopt.in

@@ -210,6 +210,9 @@ rpmbuild alias --with		--define "_with_!#:+     --with-!#:+" \
 rpmbuild alias --without	--define "_without_!#:+  --without-!#:+" \
 	--POPTdesc=$"disable configure <option> for build" \
 	--POPTargs=$"<option>"
+rpmbuild alias --scm		--define '__scm !#:+' \
+	--POPTdesc=$"shortcut for '--define=\"__scm <scm>\"'" \
+	--POPTargs=$"<scm>"
 # Build policies enabled from command line. Last policy applies.
 rpmbuild alias --buildpolicy --define '__os_install_post %{_rpmconfigdir}/brp-!#:+' \
 	--POPTdesc=$"set buildroot <policy> (e.g. compress man pages)" \