The idea is to use libo_MINGW_CHECK_DLL for libs that must be available
(typically that would be the "main" library, e.g., libxml2 or libcurl)
and libo_MINGW_TRY_DLL for possible dependencies (that may not be the
same on different systems). All further references to the dlls are
exclusively through the configured variables or defines set from these
variables (e.g., instead of hardcoding libxml2-2.dll, use
$(MINGW_LIBXML2_DLL)).

The macros are documented in m4/mingw.m4 .

Files that must be changed when adding a new dll:
* configure.in
* config_host.mk.in
* external/mingw-dlls/makefile.mk
* scp2/source/ooo/makefile.mk
* scp2/source/ooo/mingw_dlls.scp