Detection of timegm availability fails in webkit sources

The following file has problems when building Qt's webkit module from source with LSB compilers:

3rdparty/javascriptcore/JavaScriptCore/wtf/DateMath.cpp

Specifically, the compiler complains that timegm() is not availble:

../3rdparty/javascriptcore/JavaScriptCore/wtf/DateMath.cpp: In function 'int32_t QTWTF::calculateUTCOffset()':
../3rdparty/javascriptcore/JavaScriptCore/wtf/DateMath.cpp:408: error: 'timegm' was not declared in this scope

Looking at the DateMath.cpp source, this line of code should not have been compiled, since it is immediately preceded by a test for timegm availability:

#if HAVE(TIMEGM)
time_t utcOffset = timegm(&localt) - mktime(&localt);
#else
// Using a canned date of 01/01/2009 on platforms with weaker date-handling foo.
localt.tm_year = 109;
time_t utcOffset = 1230768000 - mktime(&localt);
#endif

It would seem that Qt's configure step is not recognising that timegm() is not available with the linux-lsb-g++ mkspec and/or this is not being passed through to the webkit part of the build.

Note also that the man page for timegm() describes an alternative for timegm() implemented in terms of the LSB-compliant mktime() function, so the webkit source could probably be cleaned up to make use of this as a more robust solution to the problem.