Make early-built parts shareable whether system GCC is used or not

Currently, we could just list gettext but for maintainability,
it's easier to compare the list with only python&gcc's buildout.cfg

component/defaults.cfg

 [buildout]
 extends =
+  gcc/buildout.cfg
   python-2.7/buildout.cfg
   python3/buildout.cfg
 python = python
@@ -25,6 +26,12 @@ update =
 depends = ${gcc:recipe}
 
 [gcc]
+depends =
+# requirements that are common to gcc & python
+  ${gettext:recipe}
+  ${perl:recipe}
+# python requirements for which the compiler does not matter
+  ${patch:recipe}
 recipe = slapos.recipe.build
 # Latest version provided by SlapOS.
 part = gcc-8.2

component/python3/buildout.cfg

@@ -12,7 +12,6 @@ extends =
   ../patch/buildout.cfg
   ../readline/buildout.cfg
   ../sqlite3/buildout.cfg
-  ../gcc/buildout.cfg
   ../xz-utils/buildout.cfg
   ../zlib/buildout.cfg