cmake: Generate and install a pkg-config file

I'm trying to build mtxclient, which expects a nlohmann_json pkg-config
file to exist in the system.  The meson builds install a pkg-config
file, but the cmake builds don't.  This adds a pkg-config file to the
cmake builds that is functionally equivalent to the one generated from
meson.

Signed-off-by: Palmer Dabbelt <palmer@dabbelt.com>

CMakeLists.txt

@@ -4,7 +4,7 @@ cmake_minimum_required(VERSION 3.1)
 ## PROJECT
 ## name and version
 ##
-project(nlohmann_json VERSION 3.7.0 LANGUAGES CXX)
+project(nlohmann_json VERSION 3.7.0 DESCRIPTION "JSON for Modern C++" LANGUAGES CXX)
 
 ##
 ## INCLUDE
@@ -73,6 +73,12 @@ if (MSVC)
     )
 endif()
 
+# Install a pkg-config file, so other tools can find this.
+CONFIGURE_FILE(
+  "${CMAKE_CURRENT_SOURCE_DIR}/pkg-config.pc.cmake"
+  "${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}.pc"
+)
+
 ##
 ## TESTS
 ## create and configure the unit test target
@@ -128,4 +134,8 @@ if(JSON_Install)
         NAMESPACE ${PROJECT_NAME}::
         DESTINATION ${NLOHMANN_JSON_CONFIG_INSTALL_DIR}
     )
+    install(
+        FILES "${CMAKE_BINARY_DIR}/${PROJECT_NAME}.pc"
+        DESTINATION lib/pkgconfig
+    )
 endif()

pkg-config.pc.cmake

@@ -0,0 +1,4 @@
+Name: ${PROJECT_NAME}
+Description: ${PROJECT_DESCRIPTION}
+Version: ${PROJECT_VERSION}
+Cflags: -I${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_INCLUDEDIR}