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-configure.sh

@@ -0,0 +1,18 @@
+#
+# This helper is for templates using GNU configure script.
+#
+
+# This variable can be used for packages wanting to use common arguments
+# to GNU configure scripts.
+#
+export CONFIGURE_SHARED_ARGS="--prefix=/usr --sysconfdir=/etc \
+	--infodir=/usr/share/info --mandir=/usr/share/man \
+	--localstatedir=/var"
+
+do_configure() {
+	[ -z "$configure_script" ] && configure_script="./configure"
+	${configure_script} ${CONFIGURE_SHARED_ARGS} ${configure_args}
+}
+
+# GNU configure scripts use make(1) to build/install.
+. $XBPS_HELPERSDIR/gnu-makefile.sh