configure.ac: add missing sphinx error message for OSX with Homebrew system
configure.ac: enclosed in double quote entirely

PATH=`brew --prefix gettext`"/bin:\$PATH"
PATH="`brew --prefix gettext`/bin:\$PATH"
     ^<---------------------|
Merge pull request #198 from cosmo0920/doc-configure-error-msg-OSX

configure.ac: add missing sphinx error message for OSX with Homebrew system

Patch by cosmo0920. Thanks!!!