This commit adds a PYBIND11_UNSHARED_STATIC_LOCALS macro that forces a
function to have hidden visibility under gcc and gcc-compatible
compilers. gcc, in particular, needs this to to avoid sharing static
local variables across modules (which happens even under a RTLD_LOCAL
dlopen()!). clang doesn't appear to have this issue, but the forced
visibility on internal pybind functions certainly won't hurt it and icc.
This updates the workaround from #862 to use this rather than the
version-specific template.
e98d31d697