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

CMakeLists.txt

@@ -152,6 +152,20 @@ install(
     COMPONENT example_Development
 )
 
+# pkg-config
+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
 #

simdjson.pc.in

@@ -0,0 +1,11 @@
+prefix=@CMAKE_INSTALL_PREFIX@
+includedir=${prefix}/@CMAKE_INSTALL_INCLUDEDIR@
+libdir=${prefix}/@CMAKE_INSTALL_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@