remove unnecessary shell calls

The main makefile, init.d/Makefile and src/librc/Makefile all contain
several shell calls which can be handled as make conditionals. This
switches them to conditionals.

Makefile

@@ -7,20 +7,15 @@ include Makefile.inc
 SUBDIR=		conf.d etc init.d local.d man scripts sh src
 
 # Build our old net foo or not
-_OLDNET_SH=	case "${MKOLDNET}" in \
-		[Yy][Ee][Ss]) echo "net doc";; \
-		*) echo "";; \
-		esac
-_OLDNET:=	$(shell ${_OLDNET_SH})
-SUBDIR+=	${_OLDNET}
+ifeq (${MKOLDNET},yes)
+SUBDIR+=	net doc
+endif
 
 # Build pkgconfig or not
-_PKGCONFIG_SH=	case "${MKPKGCONFIG}" in \
-		[Yy][Ee][Ss]|"") echo "pkgconfig";; \
-		*) echo "";; \
-		esac
-_PKGCONFIG:=	$(shell ${_PKGCONFIG_SH})
-SUBDIR+=	${_PKGCONFIG}
+MKPKGCONFIG?=	yes
+ifeq (${MKPKGCONFIG},yes)
+SUBDIR+=	pkgconfig
+endif
 
 # We need to ensure that runlevels is done last
 SUBDIR+=	runlevels