rework pam, ncurses and termcap include files

This allows error checking inside the included files instead of either
including the appropriate file or a blank file.

Also the blank file named .mk gets removed by this change.

mk/ncurses.mk

@@ -1,3 +0,0 @@
-LIBTERMCAP?=	-lncurses
-CPPFLAGS+=	-DHAVE_TERMCAP
-LDADD+=		${LIBTERMCAP}

mk/pam.mk

@@ -1,6 +1,10 @@
+ifeq (${MKPAM},pam)
 LIBPAM?=	-lpam
 CPPFLAGS+=	-DHAVE_PAM
 LDADD+=		${LIBPAM}
 
 PAMDIR?=	/etc/pam.d
 PAMMODE?=	0644
+else ifneq (${MKPAM},)
+$(error if MKPAM is defined, it must be "pam")
+endif

mk/termcap.mk

@@ -1,3 +1,11 @@
+ifeq (${MKTERMCAP},ncurses)
+LIBTERMCAP?=	-lncurses
+CPPFLAGS+=	-DHAVE_TERMCAP
+LDADD+=		${LIBTERMCAP}
+else ifeq (${MKTERMCAP},termcap)
 LIBTERMCAP?=	-ltermcap
 CPPFLAGS+=	-DHAVE_TERMCAP
 LDADD+=		${LIBTERMCAP}
+else ifneq (${MKTERMCAP},)
+$(error If MKTERMCAP is defined, it must be ncurses or termcap)
+endif