Add a before_install_package hook for e.g. patching the source tree

Author: sstephenson

bin/ruby-build

@@ -82,6 +82,7 @@ make_package() {
   shift
 
   pushd "$package_name" >&4
+  before_install_package "$package_name"
   build_package "$package_name" $*
   after_install_package "$package_name"
   fix_directory_permissions
@@ -206,6 +207,10 @@ build_package_copy() {
   cp -R . "$PREFIX_PATH"
 }
 
+before_install_package() {
+  local stub=1
+}
+
 after_install_package() {
   local stub=1
 }