There are now 4 levels of PCH support, the previous 'full' level
adding to PCH whatever the update_pch script finds useful,
and new levels 'system', which adds only external headers,
'base', which is 'system' and LO basic headers (sal, osl, rtl, vcl)
and 'normal', which is 'full' without headers from the module
built itself.

With Clang/GCC even 'system' still saves some time (10-15%) and since external
headers should rarely if even change, it should be without most
of the disadvantages of PCH. And even 'base' should be pretty easy
to use, as those headers should be rarely changed while developing,
thus avoiding the need for massive rebuilds. Using 'normal' or 'full'
does not seem to be worth it with Clang or GCC, but with MSVC that still
makes a difference, so keep(?) 'full' the default there.

The update_pch script unfortunately does not include as many system
headers as it could, since it includes only what is directly included
by the .cxx, but not what's included indirectly by .hxx files.

https://lists.freedesktop.org/archives/libreoffice/2019-May/082685.html

Change-Id: If83a07a1fc9b77d0134502b0d89348944f82806b
Reviewed-on: https://gerrit.libreoffice.org/71580
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>