[Development] Precompiled headers enabled for most modules (if you enabled precompiled headers)

[Thiago Macieira]

Hello

This is your friendly note that all Qt modules that don't have C or assembly 
code are now using precompiled headers. I've tested this solution for a couple 
of months with GCC on Linux, Clang on Linux, Clang on Mac and the Intel 
compiler on Linux.

As it turns out, I didn't test it in all modules with Visual Studio. I've just 
fixed one problem in QtScript and we have one left in qtlocation for QtLocation 
which makes no sense to me and I can't reproduce.

This change does *not* enable precompiled headers by default. You still need 
to pass -pch to the configure script or to configure.exe in order to get them. 
It simply creates a precompiled header for the modules that didn't previously 
define one, unless explicitly overridden.

When you should override:
 - when you have C, Objective C (not Objective C++) or assembly sources
   [done for all current modules]
 - when you need to do ugl^H^H^Hclever hacks with preprocessing that not even   
   NO_PCH_SOURCES can help
   [no known instance]
 - when you have a better precompilation source than the default

-- 
Thiago Macieira - thiago.macieira (AT) intel.com
  Software Architect - Intel Open Source Technology Center