xbps-src: revamped build_style.

By default now it's assumed that if $build_style is not set, the template
uses 'custom-install' build, .e.g do_{build,configure,install}.

If it's set, a helper with the same name with .sh extension will be sourced
to set do_{build,configure,install} phases.

The exception is "meta-template" which currently it must be set via
build_style, probably will change in the future.

xbps-src/helpers/gnu-makefile.sh

@@ -0,0 +1,26 @@
+#
+# This helper is for templates using GNU Makefiles.
+#
+do_build() {
+	if [ -z "$make_cmd" ]; then
+		make_cmd=make
+	fi
+	if [ -n "$XBPS_MAKEJOBS" -a -z "$disable_parallel_build" ]; then
+		makejobs="-j$XBPS_MAKEJOBS"
+	fi
+	${make_cmd} ${makejobs} ${make_build_args} ${make_build_target}
+}
+
+do_install() {
+	local target
+
+	if [ -z "$make_install_target" ]; then
+		target="DESTDIR=${DESTDIR} install"
+	else
+		target="${make_install_target}"
+	fi
+	if [ -z "$make_cmd" ]; then
+		make_cmd=make
+	fi
+	${make_cmd} ${make_install_args} ${target}
+}