Unexpected boost include capture.

Reported bug:

./configure --with=/usr/local/include/boost

can lead to a link error when running the message compiler.

In src/lib/util/strutil.cc with a bogus C++11 regex library (e.g with g++ 4.8):

#include <regex.h>

does not include the /usr/include/regex.h header file but /usr/local/include/boost/regex.h.

As it defined BOOST_RE_REGEX_H I suggest to check for it.