xbps-src: common/{,environment}/build_style -> common/${,environment}/build-style.

common/build-style/README

@@ -0,0 +1,12 @@
+BUILD STYLES
+============
+
+These shell snippets provide support for multiple build systems, i.e GNU configure,
+CMake, etc. A build style file must provide at least the following functions:
+
+	- do_configure
+	- do_build
+	- do_install
+
+If a source package defines its own do_xxx() function, the function defined in
+the build style file is simply ignored.