Avoid touching CFLAGS unless explicitly requested (#352)

bin/ruby-build

@@ -378,8 +378,13 @@ build_package_standard() {
   local PACKAGE_MAKE_OPTS_ARRAY="${package_var_name}_MAKE_OPTS_ARRAY[@]"
   local PACKAGE_CFLAGS="${package_var_name}_CFLAGS"
 
-  { CFLAGS="$CFLAGS ${!PACKAGE_CFLAGS}" ${!PACKAGE_CONFIGURE:-./configure} --prefix="${!PACKAGE_PREFIX_PATH:-$PREFIX_PATH}" $CONFIGURE_OPTS ${!PACKAGE_CONFIGURE_OPTS} "${!PACKAGE_CONFIGURE_OPTS_ARRAY}"
-    "$MAKE" $MAKE_OPTS ${!PACKAGE_MAKE_OPTS} "${!PACKAGE_MAKE_OPTS_ARRAY}"
+  ( if [ "${CFLAGS+defined}" ] || [ "${!PACKAGE_CFLAGS+defined}" ]; then
+      export CFLAGS="$CFLAGS {$!PACKAGE_CFLAGS}"
+    fi
+    ${!PACKAGE_CONFIGURE:-./configure} --prefix="${!PACKAGE_PREFIX_PATH:-$PREFIX_PATH}" $CONFIGURE_OPTS ${!PACKAGE_CONFIGURE_OPTS} "${!PACKAGE_CONFIGURE_OPTS_ARRAY}"
+  ) >&4 2>&1
+
+  { "$MAKE" $MAKE_OPTS ${!PACKAGE_MAKE_OPTS} "${!PACKAGE_MAKE_OPTS_ARRAY}"
     "$MAKE" install
   } >&4 2>&1
 }