configure.ac: add AC_ARG_WITH for gflags

[GoogleCodeExporter]

Automagic dependencies (where the program automatically enables/disables 
features based on the result of configure tests) aren't cool: 
http://www.gentoo.org/proj/en/qa/automagic.xml

It's quite alright to default to automagic dependencies, but if the user 
specifies --with-gflags or --without-gflags, the correct behaviour is to abort 
configure if the flags library wasn't found (--with-gflags) or to never use it, 
even if it exists (--without-gflags).

This is particularly important on source-based distributions like gentoo, where 
optional dependencies are much more common.

Original issue reported on code.google.com by endgame....@gmail.com on 22 Mar 2011 at 11:41

[GoogleCodeExporter]

Original comment by se...@google.com on 23 Mar 2011 at 1:11

• Changed state: Started

[GoogleCodeExporter]

Fixed in r9.

Original comment by se...@google.com on 23 Mar 2011 at 11:14

• Changed state: Fixed