Visibility support for Clang was disabled in 7198c33 because of linking
errors when building shared libraries, related to non-inline methods in
exported template classes.
The last Clang version to have these problems is version "4.1
(tags/Apple/clang-421.11.66)" as part of Xcode 4.5.2, and the next
release fixes them (version "4.2 (clang-425.0.24)", Xcode 4.6). Check
for the Clang version and only disable visibility support when compiling
with Apple Clang < 4.2.
Note that Apple Clang 4.2 is based on official Clang 3.2 so also check
for that version in case of non-Apple builds. The only official Clang
that has been tested successfully is 3.4 which worked fine with
visibility. Other versions that were tried (3.1 and 3.2) segfault while
compiling wx.
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