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

Respect the user's choice of shell when running mklib rather than always
using /bin/sh.

src/glw/Makefile

@@ -49,7 +49,7 @@ clean:
 
 # Make the library
 $(TOP)/$(LIB_DIR)/$(GLW_LIB_NAME): $(OBJECTS)
-	$(TOP)/bin/mklib -o $(GLW_LIB) -linker '$(CC)' -ldflags '$(LDFLAGS)' \
+	$(MKLIB) -o $(GLW_LIB) -linker '$(CC)' -ldflags '$(LDFLAGS)' \
 		-major $(MAJOR) -minor $(MINOR) -patch $(TINY) \
 		$(MKLIB_OPTIONS) -install $(TOP)/$(LIB_DIR) \
 		$(GLW_LIB_DEPS) $(OBJECTS)