fix(cmake): add required emscripten flags (#5298)

* fix(cmake): add required emscripten flags

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* Update emscripten.yaml

* fix(cmake): add required emscripten flags to headers target

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* fix(cmake): incorrect detection of Emscripten

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* fix(cmake): allow pybind11::headers to be modified

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* fix(cmake): hide a warning when building the tests standalone

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* fix(cmake): use explicit variable for is config

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* fix(cmake): go back to ALIAS target

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* chore: reduce overall diff

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* chore: reduce overall diff

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

* chore: shorten code a bit

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

---------

Signed-off-by: Henry Schreiner <henryschreineriii@gmail.com>

tools/pybind11Config.cmake.in

@@ -84,7 +84,7 @@ you can either use the basic targets, or use the FindPython tools:
 
   # Python method:
   Python_add_library(MyModule2 src2.cpp)
-  target_link_libraries(MyModule2 pybind11::headers)
+  target_link_libraries(MyModule2 PUBLIC pybind11::headers)
   set_target_properties(MyModule2 PROPERTIES
                                   INTERPROCEDURAL_OPTIMIZATION ON
                                   CXX_VISIBILITY_PRESET ON