gbuild: call gb_ExternalExecutable_collect_registrations...

...after reading platform makefile because at least python uses platform
dependent gb_PYTHON_PRECOMMAND.

Change-Id: I894dfd56b4a96828556f657ae63c6f69eebdea0b

solenv/gbuild/gbuild.mk

@@ -168,7 +168,6 @@ $(eval $(call gb_Helper_init_registries))
 include $(SRCDIR)/Repository.mk
 include $(SRCDIR)/RepositoryExternal.mk
 $(eval $(call gb_Helper_collect_knownlibs))
-$(eval $(call gb_ExternalExecutable_collect_registrations))
 
 gb_Library_DLLPOSTFIX := lo
 
@@ -177,6 +176,9 @@ include $(GBUILDDIR)/platform/$(OS)_$(CPUNAME)_$(COM).mk
 
 include $(SRCDIR)/RepositoryFixes.mk
 
+# after platform; at least currently python depends on variable set in platform
+$(eval $(call gb_ExternalExecutable_collect_registrations))
+
 # add user-supplied flags
 ifneq ($(strip gb__ENV_CFLAGS),)
 gb_LinkTarget_CFLAGS += $(gb__ENV_CFLAGS)