[lmi] precompiled headers in LMI

[Vadim Zeitlin]

 Hello,

 While working on MSVC compilation I noticed that LMI sources mention the
symbol LMI_COMPILER_USES_PCH and there already exists a pchfile.hpp file.
However this file is only included for Borland C++ compiler as it's guarded
by "#ifdef __BORLANDC__" and not "#ifdef LMI_COMPILER_USES_PCH" (or nothing
at all) as I'd expect and also not included from all files while both
Borland and MSVC normally need all files to include the PCH header if it's
used. To be precise, at least with MSVC you can use different PCH options
for different files in the project but this would make the MSVC project
files difficult to generate automatically.

 Anyhow, at this point I'd just like to know if you think this is something
worth spending time on? Of course, PCH support is not limited to MSVC and
Borland compilers, recent (>= 3.4) g++ versions support them as well, and
while cygwin has some bugs related to them I don't think they affect LMI
and, anyhow, these bugs will be fixed sooner or later and PCH support would
be already usable under Unix systems.

 And, while you certainly are already aware of it, I'd just like to remind
that the benefit of PCH is that judicious inclusion of the mostly common
used headers in pchfile.hpp (or, probably, lmi_wx_pch.hpp and other files
for each target) can dramatically reduce the compilation time, e.g. factor
of 10 is not uncommon.

 So what do you think of modifying LMI sources to use them?

 Thanks,
VZ