Find python3 via pkg-config when available.

This is needed when using linuxdeploy / appimagecraft to create an AppImage
and when cross compiling (f.e. with MXE) to find the python libraries.
You may need to set CMAKE_FIND_ROOT_PATH_MODE_LIBRARY to BOTH when cross
compiling.

This may fix issues pybind#1718, pybind#1159, pybind#1330 and pybind#99.

Author: knarfS

CMakeLists.txt

@@ -164,6 +164,7 @@ target_include_directories(
   pybind11 ${pybind11_system} INTERFACE $<BUILD_INTERFACE:${PYBIND11_INCLUDE_DIR}>
                                         $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
 # Only add Python for build - must be added during the import for config since it has to be re-discovered.
+# TODO: isn't working, when PYTHON_INCLUDE_DIRS contains a semikolon seperated list? -> ${PYTHON_INCLUDE_DIRS}
 target_include_directories(pybind11 SYSTEM INTERFACE $<BUILD_INTERFACE:${PYTHON_INCLUDE_DIRS}>)
 
 if(CMAKE_VERSION VERSION_LESS 3.13)

tools/pybind11Tools.cmake

@@ -17,9 +17,26 @@ set(Python_ADDITIONAL_VERSIONS
     "3.9;3.8;3.7;3.6;3.5;3.4"
     CACHE INTERNAL "")
 
-list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_LIST_DIR}")
-find_package(PythonLibsNew ${PYBIND11_PYTHON_VERSION} MODULE REQUIRED)
-list(REMOVE_AT CMAKE_MODULE_PATH -1)
+# Try to get python via pgk-config, else fall back to the PythonLibsNew script.
+# When cross compiling, you need to set CMAKE_FIND_ROOT_PATH_MODE_LIBRARY to BOTH.
+find_package(PkgConfig)
+if (PkgConfig_FOUND)
+  pkg_check_modules(PYTHON3 python3)
+  if (PYTHON3_FOUND)
+    set(PYTHON_INCLUDE_DIRS ${PYTHON3_INCLUDE_DIRS})
+    set(PYTHON_LIBRARIES ${PYTHON3_LINK_LIBRARIES})
+    set(PYTHON_MODULE_PREFIX "")
+    set(PYTHON_MODULE_EXTENSION "")
+    set(PYTHON_VERSION_MAJOR "") # TODO
+    set(PYTHON_VERSION_MINOR "") # TODO
+    message(STATUS "Python3 found via pkg-config!")
+  endif()
+endif()
+if (NOT PYTHON3_FOUND)
+  list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_LIST_DIR}")
+  find_package(PythonLibsNew ${PYBIND11_PYTHON_VERSION} MODULE REQUIRED)
+  list(REMOVE_AT CMAKE_MODULE_PATH -1)
+endif()
 
 include(CheckCXXCompilerFlag)