Python: add_library -> Python_add_library

Using this function implements a fallback for systems where no static
library support exists.

Examples: old Cray systems and emscripten (WebAssembly).

CMakeLists.txt

@@ -118,7 +118,7 @@ if(DEFINED BUILD_SHARED_LIBS)
 else()
     set(openPMD_BUILD_SHARED_LIBS_DEFAULT ${SHARED_LIBS_SUPPORTED})
 endif()
-option(openPMD_BUILD_SHARED_LIBS "Build the openPMD tests"
+option(openPMD_BUILD_SHARED_LIBS "Build shared openPMD libraries"
     ${openPMD_BUILD_SHARED_LIBS_DEFAULT})
 if(openPMD_BUILD_SHARED_LIBS AND NOT SHARED_LIBS_SUPPORTED)
     message(FATAL_ERROR "openPMD_BUILD_SHARED_LIBS requested but not supported by platform")
@@ -375,6 +375,9 @@ if(CMAKE_VERSION VERSION_LESS 3.18.0)
     #   https://cmake.org/cmake/help/v3.18/module/FindPython.html
     set(_PY_DEV_MODULE Development)
 endif()
+if(EMSCRIPTEN)
+    set(_PY_DEV_MODULE ${_PY_DEV_MODULE} Development.Embed)
+endif()
 if(openPMD_USE_PYTHON STREQUAL AUTO)
     find_package(Python 3.6.0 COMPONENTS Interpreter ${_PY_DEV_MODULE})
     if(Python_FOUND)
@@ -640,7 +643,7 @@ endif()
 
 # python bindings
 if(openPMD_HAVE_PYTHON)
-    add_library(openPMD.py MODULE
+    Python_add_library(openPMD.py MODULE
         src/binding/python/openPMD.cpp
         src/binding/python/Access.cpp
         src/binding/python/Attributable.cpp