Relax check for "~" sign in DESTDIR to not fail with unreleased builds

Using git-buildpackage for continuous integration we have
something like:

| /usr/bin/make DESTDIR=/tmp/buildd/ngcp-asterisk-1.4.24.1+ngcp.1+0~1367942255.82+squeeze~1.gbp58b891/debian/ngcp-asterisk/ install

in some of our build processes. To avoid failing at this stage
modify the check in the "badshell" make target to check for "~/"
instead of just checking for "~" anywhere inside the version
string.

Also see: https://issues.asterisk.org/jira/browse/ASTERISK-13797
Closes: https://jenkins.mgm.sipwise.com/job/asterisk-binaries/78/

Makefile

@@ -507,7 +507,7 @@ oldmodcheck:
 	fi
 
 badshell:
-ifneq ($(findstring ~,$(DESTDIR)),)
+ifneq ($(findstring ~/,$(DESTDIR)),)
 	@echo "Your shell doesn't do ~ expansion when expected (specifically, when doing \"make install DESTDIR=~/path\")."
 	@echo "Try replacing ~ with \$$HOME, as in \"make install DESTDIR=\$$HOME/path\"."
 	@exit 1