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/perl-module.sh

@@ -4,17 +4,15 @@
 #
 # Required vars to be set by a template:
 #
-# 	build_style=perl_module
+# 	build_style=perl-module
 #
 # Optionally if the module needs more directories to be configured other
 # than $XBPS_BUILDDIR/$wrksrc, one can use (relative to $wrksrc):
 #
 #	perl_configure_dirs="blob/bob foo/blah"
 #
-
-perl_module_build()
-{
-	local perlmkf=
+do_configure() {
+	local perlmkf
 
 	if [ -z "$perl_configure_dirs" ]; then
 		perlmkf="$wrksrc/Makefile.PL"
@@ -41,3 +39,6 @@ perl_module_build()
 		fi
 	done
 }
+
+# Perl modules use standard make(1) to install.
+. ${XBPS_HELPERSDIR}/gnu-makefile.sh