[Mingw-cross-env-list] suggested GCC patch for exporting symbols needed by boost

[Hamza Alloush]

Hello,

GCC post version 5 seems to have a bug, that prevents some symbols from 
exporting to the libstdc++, which prevents Boost from compiling. since then i 
had downgraded to GCC 4.9.2 in MXE to fix this issue.
but after posting to a ticket reporting this same issue on the bug tracker for 
GCC, they have admitted to fixing it and have since produced a patch, the patch 
is not intrusive and only adds a Mingw conditional to export some symbols 
whenever the "facet" codecvt_byname is called, which is used for code 
conversion by Boost.

you can find all the info in this report: 
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66030

i have submitted a patch for merging to the Upstream, would appreciate if you 
can look at it, test before/after Boost compile results, it is working for me 
personally now, but it might be helpful to have confirmations from other users!!

gist file:

http://pastebin.com/HcizTVkg

proposed fix attached, merge request performed.

Thanks!
Hamza

gcc.patch
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