feat(cmake): add installation support for pkg-config dependency detection (#4077)

* add installation support for pkg-config dependency detection

pkg-config is a buildsystem-agnostic alternative to
`pybind11Config.cmake` that can be used from build systems other than
cmake.

Fixes #230

* tests: add test for pkg config

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

Co-authored-by: Henry Schreiner <henryschreineriii@gmail.com>

tools/JoinPaths.cmake

@@ -0,0 +1,23 @@
+# This module provides function for joining paths
+# known from most languages
+#
+# SPDX-License-Identifier: (MIT OR CC0-1.0)
+# Copyright 2020 Jan Tojnar
+# https://github.com/jtojnar/cmake-snips
+#
+# Modelled after Python’s os.path.join
+# https://docs.python.org/3.7/library/os.path.html#os.path.join
+# Windows not supported
+function(join_paths joined_path first_path_segment)
+    set(temp_path "${first_path_segment}")
+    foreach(current_segment IN LISTS ARGN)
+        if(NOT ("${current_segment}" STREQUAL ""))
+            if(IS_ABSOLUTE "${current_segment}")
+                set(temp_path "${current_segment}")
+            else()
+                set(temp_path "${temp_path}/${current_segment}")
+            endif()
+        endif()
+    endforeach()
+    set(${joined_path} "${temp_path}" PARENT_SCOPE)
+endfunction()

tools/pybind11.pc.in

@@ -0,0 +1,7 @@
+prefix=@prefix_for_pc_file@
+includedir=@includedir_for_pc_file@
+
+Name: @PROJECT_NAME@
+Description: Seamless operability between C++11 and Python
+Version: @PROJECT_VERSION@
+Cflags: -I${includedir}

tools/setup_global.py.in

@@ -29,6 +29,7 @@ main_headers = glob.glob("pybind11/include/pybind11/*.h")
 detail_headers = glob.glob("pybind11/include/pybind11/detail/*.h")
 stl_headers = glob.glob("pybind11/include/pybind11/stl/*.h")
 cmake_files = glob.glob("pybind11/share/cmake/pybind11/*.cmake")
+pkgconfig_files = glob.glob("pybind11/share/pkgconfig/*.pc")
 headers = main_headers + detail_headers + stl_headers
 
 cmdclass = {"install_headers": InstallHeadersNested}
@@ -51,6 +52,7 @@ setup(
     headers=headers,
     data_files=[
         (base + "share/cmake/pybind11", cmake_files),
+        (base + "share/pkgconfig", pkgconfig_files),
         (base + "include/pybind11", main_headers),
         (base + "include/pybind11/detail", detail_headers),
         (base + "include/pybind11/stl", stl_headers),

tools/setup_main.py.in

@@ -17,6 +17,7 @@ setup(
         "pybind11.include.pybind11.detail",
         "pybind11.include.pybind11.stl",
         "pybind11.share.cmake.pybind11",
+        "pybind11.share.pkgconfig",
     ],
     package_data={
         "pybind11": ["py.typed"],
@@ -24,6 +25,7 @@ setup(
         "pybind11.include.pybind11.detail": ["*.h"],
         "pybind11.include.pybind11.stl": ["*.h"],
         "pybind11.share.cmake.pybind11": ["*.cmake"],
+        "pybind11.share.pkgconfig": ["*.pc"],
     },
     extras_require={
         "global": ["pybind11_global==$version"]