Force hidden visibility on functions under gcc for distinct static locals

This commit adds an attribute macro PYBIND11_UNSHARED_STATIC_LOCALS
that forces a function to have hidden visibility under gcc.  This is
needed to force gcc to avoid sharing static local instances across
modules (which happens even under a RTLD_LOCAL dlopen()!)

This updates the workaround from #862 to use this rather than the
version-specific-template.

Author: jagerman

include/pybind11/cast.h

@@ -60,12 +60,7 @@ 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 internals *&get_internals_ptr() { static internals *internals_ptr = nullptr; return internals_ptr; }
 
 PYBIND11_NOINLINE inline internals &get_internals() {
     internals *&internals_ptr = get_internals_ptr();
@@ -128,13 +123,11 @@ PYBIND11_NOINLINE inline internals &get_internals() {
     return *internals_ptr;
 }
 
-namespace {
 // Works like internals.registered_types_cpp, but for module-local registered types:
-inline type_map<void *> &registered_local_types_cpp() {
+PYBIND11_NOINLINE PYBIND11_UNSHARED_STATIC_LOCALS inline type_map<void *> &registered_local_types_cpp() {
     static type_map<void *> locals{};
     return locals;
 }
-}
 
 /// A life support system for temporary objects created by `type_caster::load()`.
 /// Adding a patient will keep it alive up until the enclosing function returns.

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 currently only has an effect under g++,
+// which forces the function visibility to "hidden" which avoids the sharing.
+#if defined(__GNUG__) && !defined(__clang__)
+#  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