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

@@ -98,7 +98,8 @@ if(NOT (CMAKE_VERSION VERSION_LESS 3.0))  # CMake >= 3.0
   add_library(pybind11 INTERFACE)
   add_library(pybind11::pybind11 ALIAS pybind11)  # to match exported target
   target_include_directories(pybind11 INTERFACE $<BUILD_INTERFACE:${PYBIND11_INCLUDE_DIR}>
-                                                $<BUILD_INTERFACE:${PYTHON_INCLUDE_DIRS}>
+                                                # $<BUILD_INTERFACE:${PYTHON_INCLUDE_DIRS}> isn't working, when PYTHON_INCLUDE_DIRS contains a semikolon seperated list
+                                                ${PYTHON_INCLUDE_DIRS}
                                                 $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>)
   target_compile_options(pybind11 INTERFACE $<BUILD_INTERFACE:${PYBIND11_CPP_STANDARD}>)
 

tools/pybind11Tools.cmake

@@ -13,7 +13,25 @@ if(NOT PYBIND11_PYTHON_VERSION)
 endif()
 
 set(Python_ADDITIONAL_VERSIONS 3.9 3.8 3.7 3.6 3.5 3.4)
-find_package(PythonLibsNew ${PYBIND11_PYTHON_VERSION} REQUIRED)
+
+# 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) # TODO PYBIND11_PYTHON_VERSION
+  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)
+  find_package(PythonLibsNew ${PYBIND11_PYTHON_VERSION} REQUIRED)
+endif()
 
 include(CheckCXXCompilerFlag)
 include(CMakeParseArguments)