Force hidden visibility on functions needing distinct static locals

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.

include/pybind11/cast.h

@@ -60,12 +60,10 @@ struct type_info {
     bool default_holder : 1;
 };
 
-// Store the static internals pointer in a version-specific function so that we're guaranteed it
-// will be distinct for modules compiled for different pybind11 versions.  Without this, some
-// compilers (i.e. gcc) can use the same static pointer storage location across different .so's,
-// even though the `get_internals()` function itself is local to each shared object.
-template <int = PYBIND11_VERSION_MAJOR, int = PYBIND11_VERSION_MINOR>
-internals *&get_internals_ptr() { static internals *internals_ptr = nullptr; return internals_ptr; }
+PYBIND11_UNSHARED_STATIC_LOCALS PYBIND11_NOINLINE inline internals *&get_internals_ptr() {
+    static internals *internals_ptr = nullptr;
+    return internals_ptr;
+}
 
 PYBIND11_NOINLINE inline internals &get_internals() {
     internals *&internals_ptr = get_internals_ptr();

include/pybind11/common.h

@@ -68,6 +68,16 @@
 #  endif
 #endif
 
+// Attribute macro for a function containing one or more static local variables that mustn't share
+// the variable across shared objects (for example, because the value might be incompatible for
+// modules compiled under different pybind versions).  This is required under g++ (depending on the
+// specific compiler and linker options), and won't hurt under gcc-compatible compilers:
+#if defined(__GNUG__)
+#  define PYBIND11_UNSHARED_STATIC_LOCALS __attribute__ ((visibility("hidden")))
+#else
+#  define PYBIND11_UNSHARED_STATIC_LOCALS
+#endif
+
 #if defined(_MSC_VER)
 #  define PYBIND11_NOINLINE __declspec(noinline)
 #else