gbuild: add var. for enabling runnable instdir

This is a part of my effort to get a runnable $(INSTDIR) as a more
convenient alternative to make dev-install. I plan to copy all necessary
shared libs, executables and other files to $(INSTDIR), which increases
the disk space demands for a build (there would be three copies of a
library/executable in the same build: one in $(WORKDIR), another in
$(OUTDIR) and one in $(INSTDIR)). Because the runnable instdir idea is
limited to Linux anyway (at least for now), it will be disabled by
default.

Change-Id: Ib61109b1a193c61c0236b50b64c34fea20f40297

solenv/gbuild/gbuild.mk

@@ -275,6 +275,8 @@ gb_GLOBALDEFS += \
 
 gb_GLOBALDEFS := $(sort $(gb_GLOBALDEFS))
 
+gb_RUNNABLE_INSTDIR :=
+
 include $(GBUILDDIR)/Deliver.mk
 
 $(eval $(call gb_Deliver_init))