...and thus exporting osl::Guard<comphelper::SolarMutex> members from vcl.  With
--disable-pch that caused linking Library_acc to fail with duplicate symbols, as
accessibility/source/standard/vclxaccessiblebutton.cxx uses
comphelper::OExternalLockGuard (include/comphelper/accessiblecontexthelper.hxx),
which also derives from osl::Guard<comphelper::SolarMutex> (and is also all-
inline, and not marked as DLLPUBLIC), so also emits such members.  With
--enable-pch, vclxaccessiblebutton.cxx happens to see SolarMutexGuard from
include/vcl/svapp.hxx before comphelper::OExternalLockGuard, and thus doesn't
emit any such members.

As SolarMutexGuard is all-inline, there should not be much point in making it
VCL_DLLPUBLIC in the first place (was there ever since the class's introduction
in b450a328 "create a class SolarMutexGuard to
take a Guard on the SolarMutex"), so just drop that.

Change-Id: Ie9d493370c7d34981bb35e5d9e100cf987eb83ca
Reviewed-on: https://gerrit.libreoffice.org/42616Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>