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/
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Merged revisions 425291 from http://svn.asterisk.org/svn/asterisk/branches/1.8
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Merged revisions 425292 from http://svn.asterisk.org/svn/asterisk/branches/11
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Merged revisions 425293 from http://svn.asterisk.org/svn/asterisk/branches/12
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Merged revisions 425294 from http://svn.asterisk.org/svn/asterisk/branches/13


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@425295 65c4cc65-6c06-0410-ace0-fbb531ad65f3