Use hidden visibility always; set via cmake property rather than compiler flag

This should resolve the issues with default visibility causing problems
under debug compilations.  Moreover using the cmake property makes it
easier for a caller to override if absolutely needed for some reason.

Author: jagerman

tools/pybind11Tools.cmake

@@ -139,6 +139,13 @@ function(pybind11_add_module target_name)
   set_target_properties(${target_name} PROPERTIES PREFIX "${PYTHON_MODULE_PREFIX}")
   set_target_properties(${target_name} PROPERTIES SUFFIX "${PYTHON_MODULE_EXTENSION}")
 
+  # -fvisibility=hidden is required to allow multiple modules compiled against
+  # different pybind versions to work properly, and for some features (e.g.
+  # py::module_local).  We force it on everything inside the `pybind11`
+  # namespace; also turning it on for a pybind module compilation here avoids
+  # potential warnings or issues from having mixed hidden/non-hidden types.
+  set_target_properties(${target_name} PROPERTIES CXX_VISIBILITY_PRESET "hidden")
+
   if(WIN32 OR CYGWIN)
     # Link against the Python shared library on Windows
     target_link_libraries(${target_name} PRIVATE ${PYTHON_LIBRARIES})
@@ -175,9 +182,6 @@ function(pybind11_add_module target_name)
   _pybind11_add_lto_flags(${target_name} ${ARG_THIN_LTO})
 
   if (NOT MSVC AND NOT ${CMAKE_BUILD_TYPE} MATCHES Debug)
-    # Set the default symbol visibility to hidden (very important to obtain small binaries)
-    target_compile_options(${target_name} PRIVATE "-fvisibility=hidden")
-
     # Strip unnecessary sections of the binary on Linux/Mac OS
     if(CMAKE_STRIP)
       if(APPLE)