build: Relax badshell tilde test to allow for ~ in middle of DESTDIR.

The main Makefile has a target test called 'badshell' that tests if
DESTDIR does not happen to have an an-expanded tilde (~).  This might
be the case if you run: make install DESTDIR=~/somewhere/

That test also disallowed valid tildes in directory names. The test is
now changed to only trigger on a tilde at the start of the path.

ASTERISK-13797 #close
Reported by: Tzafrir Cohen

Review: https://reviewboard.asterisk.org/r/4064/
........

Merged revisions 425291 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@425292 65c4cc65-6c06-0410-ace0-fbb531ad65f3

Makefile

@@ -600,7 +600,7 @@ oldmodcheck:
 	fi
 
 badshell:
-ifneq ($(findstring ~,$(DESTDIR)),)
+ifneq ($(filter ~%,$(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