Define a global %clean default, reuse for the non-buildsystem, case too

We've had this special case for %clean for ages, but this actually
offers a nice opportunity to put it into a macro that allows customization
and is used for more than one purpose.

build/parseSpec.c

@@ -1214,7 +1214,7 @@ static rpmSpec parseSpec(const char *specFile, rpmSpecFlags flags,
 	goto errxit;
 
     if (spec->sections[SECT_CLEAN] == NULL) {
-	char *body = rpmExpand("%{?buildroot: %{__rm} -rf %{buildroot}}", NULL);
+	char *body = rpmExpand("%{buildsystem_default_clean}", NULL);
 	spec->sections[SECT_CLEAN] = newStringBuf();
 	appendLineStringBuf(spec->sections[SECT_CLEAN], body);
 	free(body);

macros.in

@@ -1354,6 +1354,7 @@ end
 
 # Global buildsystem defaults
 %buildsystem_default_prep() %autosetup -p1 %*
+%buildsystem_default_clean() %{__rm} -rf %{buildroot}
 
 # Example buildsystem for autotools
 %buildsystem_autotools_conf() %configure %*