Added a FAQ entry regarding: "'system_error' file not found".
Added a FAQ entry regarding the C++11 compiler autodetection.
Ensured that compiler autodetection prefers Clang over GCC.
Ensured that AC_USE_SYSTEM_EXTENSIONS is used only after AC_PROG_CC.

The converse breaks AC_PROG_CC's explicit compiler precedence list.
Added Ohloh, Freecode, SourceForge, and Travis CI links to the README.
Bumped the Autoconf build dependency to 2.68 (released 2010-09-22).

We had already inadvertently been using AC_INIT with the AC_PACKAGE_URL
argument, necessitating 2.64+. Looking over the Autoconf change log,
there were enough significant regression bugs in the several releses
prior to 2.68 to warrant requiring it as a known-good baseline.

See: http://git.savannah.gnu.org/gitweb/?p=autoconf.git;a=blob_plain;f=NEWS;hb=HEAD