Fix compiler error when using ./configure --enable-dev-mode --enable-coverage

When DONT_OPTIMIZE is enabled with dev-mode, it causes a shadow compilation
to be done with output to /dev/null.  This can cause errors with coverage
when GCC attempts to write to /dev/null.gcno.  This change disables
coverage for the shadow compilation.

ASTERISK-24502 #close
Reported by: Corey Farrell
Review: https://reviewboard.asterisk.org/r/4151/
........

Merged revisions 427682 from http://svn.asterisk.org/svn/asterisk/branches/11


git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@427683 f38db490-d61c-443f-a65b-d21fe96a405b