build: add pkg-config support (#1767)

* build: add pkg-config support

The CMake build script now generates a simple pkg-config files that can
be easily used by non-CMake users.

The file is generated from a template file that gets filled in at
configure time.

As CMake doesn't have anything similar to Meson's pkg-config generator
the file is quite static, i.e. new simdjson public defines/dependencies
won't be picked up automatically.

This approach also suffers from one minor issue, mentioned in
[jtojnar/cmake-snips][]; in short, it doesn't work well when users
specify CMAKE_INSTALL_INCLUDEDIR and similar as absolute paths. It's not
a big deal, and it will easily fixable once you'll require CMake >=3.20.

Fixes #1763

[jtojnar/cmake-snips]: https://github.com/jtojnar/cmake-snips#concatenating-paths-when-building-pkg-config-files

* build: handle absolute paths in .pc generation

As mentioned in the previous commit message, correct concatenation of
paths is only available in CMake >=3.20, so handling absolute paths in
pkg-config file generation requires using jtojnar's JoinPaths module.

* ci: add debian job

This new jobs compiles simdjson on Debian Testing, a semi-rolling
release, so that new compilers are always tested.

This job also tests the pkg-config file introduced in commit
1096c3b299

.github/workflows/debian.yaml

@@ -0,0 +1,33 @@
+name: Debian
+
+on: [push, pull_request]
+
+defaults:
+  run:
+    shell: sh
+
+permissions:
+  contents: read
+
+jobs:
+  pkg-config:
+    runs-on: ubuntu-latest
+    container:
+      image: debian:testing
+
+    steps:
+    - uses: actions/checkout@v3
+
+    - name: Install dependencies
+      run: |
+        apt -y update
+        apt -y --no-install-recommends install g++ cmake make pkg-config
+
+    - name: Build and install
+      run: |
+        cmake -B build
+        cmake --build build
+        cmake --install build
+
+    - name: Test pkg-config
+      run: g++ examples/quickstart/quickstart.cpp $(pkg-config --cflags --libs simdjson)

CMakeLists.txt

@@ -152,6 +152,24 @@ install(
     COMPONENT example_Development
 )
 
+# pkg-config
+include(cmake/JoinPaths.cmake)
+join_paths(PKGCONFIG_INCLUDEDIR "\${prefix}" "${CMAKE_INSTALL_INCLUDEDIR}")
+join_paths(PKGCONFIG_LIBDIR "\${prefix}" "${CMAKE_INSTALL_LIBDIR}")
+
+if(SIMDJSON_ENABLE_THREADS)
+  set(PKGCONFIG_CFLAGS "-DSIMDJSON_THREADS_ENABLED=1")
+  if(CMAKE_THREAD_LIBS_INIT)
+    set(PKGCONFIG_LIBS_PRIVATE "Libs.private: ${CMAKE_THREAD_LIBS_INIT}")
+  endif()
+endif()
+
+configure_file("simdjson.pc.in" "simdjson.pc" @ONLY)
+install(
+    FILES "${CMAKE_CURRENT_BINARY_DIR}/simdjson.pc"
+    DESTINATION "${CMAKE_INSTALL_LIBDIR}/pkgconfig"
+)
+
 #
 # CPack
 #

cmake/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()

simdjson.pc.in

@@ -0,0 +1,11 @@
+prefix=@CMAKE_INSTALL_PREFIX@
+includedir=@PKGCONFIG_INCLUDEDIR@
+libdir=@PKGCONFIG_LIBDIR@
+
+Name: @PROJECT_NAME@
+Description: @PROJECT_DESCRIPTION@
+URL: @PROJECT_HOMEPAGE_URL@
+Version: @PROJECT_VERSION@
+Cflags: -I${includedir} @PKGCONFIG_CFLAGS@
+Libs: -L${libdir} -l@PROJECT_NAME@
+@PKGCONFIG_LIBS_PRIVATE@