This is a variant of Input for layout files that only searches build
and system directories and thus allows "InputGlobal name.inc" in a user
file name.inc that attempts to modify its global counterpart.
At the moment, there is no support for plurals and capitalization.
A long comment explains why. Support could be added for that without
a format change.
References to environment variables embedded in a filename are expanded
and replaced by their value. However, if a variable does not exist, its
reference is simply erased from the filename, causing havoc (see #7801).
This has been like that since ever and cannot be changed, both for
backward compatibility and because this feature is currently used in
the Windows installer.
A possible backward compatible strategy is leaving as is the reference
to the environment variable (introduced by a $ sign) in the filename
if it does not exist. This is done in this patch, which also assumes
that an escape character is never used in a filename (inserting a $ in
the filename is easy, but I don't think one is able to easily insert
an escape character).
boost/signals2.hpp is a convenience header, it seems that
boost/signals2/signal.hpp is enough for our needs.
This allows to get rid of 6MiB of header files in 3rdparty/boost (and
probably to speed-up compilation).
1/ remove boost regex code from 3rdparty/boost. Only the cmake stuff remains.
2/ remove traces of BOOST_LIB since there is no need anymore to
compile boost stuff.
3/ remove traces of boost regexp in config/lyxinclude.m4 and INSTALL
4/ require gcc 4.9 as minimal version in autoconf.
Unfortunately, the net gain is only 10% of included boost size (now 21M).
