Set MKFLAGS to MAKEFLAGS= (the magic bit), per Deuce.

A second fix for the recursive make issue in src/sbbs3/targets.mk
(build of scfg, uedit, and umonitor).

install/GNUmakefile

@@ -30,6 +30,8 @@
 # NO_GTK = Don't build GTK-based sysop tools
 # X_PATH = /path/to/X (if not /usr/X11R6)
 
+# the magic bit:
+MKFLAGS += MAKEFLAGS=
 ifndef DEBUG
  ifndef RELEASE
   RELEASE	:=	1