Set $(SHELL) for all configs

Most make implementations will use /bin/sh as the interpreter for
commands and only use a different shell when the $(SHELL) make variable
is set. This makes the setting explicit and allows $(SHELL) to be used
in the commands themselves.
diff --git a/configs/default b/configs/default
index d5b40c5..5193c14 100644
--- a/configs/default
+++ b/configs/default
@@ -25,6 +25,7 @@
 GLU_CFLAGS = 
 
 # Misc tools and flags
+SHELL = /bin/sh
 MKLIB_OPTIONS = 
 MKDEP = makedepend
 MKDEP_OPTIONS = -fdepend