bug#41152: DLL_EXPORT in libtool.m4

[Malcolm MacLeod]

When building a shared library (dll) on windows (msys2; but also other 
environments) using libtool,
libtool.m4 defines -DDLL_EXPORT - which is mentioned in libtool.m4 as being a 
'hack'.

In a normal situation this doesn't have an obvious ill effect, but if you are 
attempting to build a
dll whose linkage to all other libraries (e.g. pthread) is static (as is 
sometimes desirable if e.g.
building a plugin) this results in all object files being contaminated with 
symbols for dynamic
pthread which then fail at link time, because of course pthread is only 
statically linked.

I believe this hack does more harm than good and should be removed, it is 
common practice on windows
that if you want exports/imports you should request them by setting the define 
yourself, so I don't
see why libtool attempts to automatically set this.
Should it not be possible to remove the hack, there should at least be some way 
to turn it off,
currently I have to now "#undef DLL_EXPORT" all over my codebase which is messy 
and undesirable.

Regards,
Malcolm MacLeod