Call minstall with $(SHELL) so the user controls the interpreter

Running minstall directly means that /bin/sh is always used as hte
interpreter. If the user needs or wants to use a different shell fo
minstall, they can use the SHELL make variable.
diff --git a/configs/default b/configs/default
index 5193c14..6c2594b 100644
--- a/configs/default
+++ b/configs/default
@@ -30,7 +30,7 @@
 MKDEP = makedepend
 MKDEP_OPTIONS = -fdepend
 MAKE = make
-INSTALL = $(TOP)/bin/minstall
+INSTALL = $(SHELL) $(TOP)/bin/minstall
 
 # Tools for regenerating glapi (generally only needed by the developers)
 PYTHON2 = python