Fix FREESTANDING install target

When installing a FREESTANDING build, the check for an empty variable
was not being honored because the empty string would evaluate to
nothing.  Put the variable expansion in quotes so that we have an empty
string instead.

Makefile

@@ -215,7 +215,7 @@ install: all
 		install -D -m644 $$i ${DESTDIR}${includedir}/$$destfn; \
 	done
 	install -D -m644 ${LIBUCONTEXT_PC} ${DESTDIR}${pkgconfigdir}/${LIBUCONTEXT_PC}
-	if [ -n ${LIBUCONTEXT_POSIX_NAME} ]; then \
+	if [ -n "${LIBUCONTEXT_POSIX_NAME}" ]; then \
 		install -D -m755 ${LIBUCONTEXT_POSIX_NAME} ${DESTDIR}${LIBUCONTEXT_POSIX_PATH}; \
 		install -D -m644 ${LIBUCONTEXT_POSIX_STATIC_NAME} ${DESTDIR}${LIBUCONTEXT_POSIX_STATIC_PATH}; \
 	fi