add BLOCK_PCH to gbuild, allowing partial non-PCH rebuilds

Both MSVC and Clang (with -building-pch-with-obj) generate one extra
object file for code from the PCH, saving repeated generating of this
code for every object using the PCH. This causes problems when
temporarily disabling PCH by doing 'make ENABLE_PCH=' (e.g. when
checking #include's are correct), as this object would no longer be
linked in, and objects not rebuilt with PCH disabled would still need
it.
This patch allows doing 'make BLOCK_PCH=1' instead, which will disable
PCH with the exception of this object file still getting linked in.

Change-Id: I8fcb150ef27ebb34118d62739a1a1558aa1a6f3d
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/88341
Tested-by: Jenkins
Reviewed-by: Luboš Luňák <l.lunak@collabora.com>

0 files changed, 0 insertions, 0 deletions