Add generation of registry file for pkg-config (#151)

.pkg-config/taocpp-json.pc.in

@@ -0,0 +1,7 @@
+includedir="@CMAKE_INSTALL_FULL_INCLUDEDIR@"
+
+Name: @PROJECT_NAME@
+Description: taoJSON is a C++ header-only JSON library
+Version: @PROJECT_VERSION@
+URL: https://github.com/taocpp/json
+Cflags: -I${includedir}

CMakeLists.txt

@@ -12,6 +12,7 @@ include(GNUInstallDirs)
 
 set(TAOCPP_JSON_INSTALL_DOC_DIR "${CMAKE_INSTALL_DOCDIR}/tao/json" CACHE STRING "The installation doc directory")
 set(TAOCPP_JSON_INSTALL_CMAKE_DIR "${CMAKE_INSTALL_DATAROOTDIR}/${PROJECT_NAME}/cmake" CACHE STRING "The installation cmake directory")
+set(TAOCPP_JSON_INSTALL_PKGCONFIG_DIR "${CMAKE_INSTALL_DATAROOTDIR}/pkgconfig" CACHE STRING "The installation pkgconfig directory")
 
 # define a header-only library
 add_library(taocpp-json INTERFACE)
@@ -61,13 +62,20 @@ option(TAOCPP_JSON_INSTALL "Generate the install target" ${TAOCPP_JSON_IS_MAIN_P
 if(TAOCPP_JSON_INSTALL)
   include(CMakePackageConfigHelpers)
 
-  # Make package findable
+  # Make package findable by CMake
   configure_package_config_file(.cmake/taocpp-json-config.cmake.in ${PROJECT_NAME}-config.cmake
     INSTALL_DESTINATION ${TAOCPP_JSON_INSTALL_CMAKE_DIR}
     NO_CHECK_REQUIRED_COMPONENTS_MACRO
     NO_SET_AND_CHECK_MACRO
   )
 
+  # Make package findable by pkg-config
+  configure_package_config_file(.pkg-config/taocpp-json.pc.in ${PROJECT_NAME}.pc
+    INSTALL_DESTINATION ${TAOCPP_JSON_INSTALL_PKGCONFIG_DIR}
+    NO_CHECK_REQUIRED_COMPONENTS_MACRO
+    NO_SET_AND_CHECK_MACRO
+  )
+
   # Ignore pointer width differences since this is a header-only library
   unset(CMAKE_SIZEOF_VOID_P)
 
@@ -84,6 +92,10 @@ if(TAOCPP_JSON_INSTALL)
     ${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}-config-version.cmake
     DESTINATION ${TAOCPP_JSON_INSTALL_CMAKE_DIR}
   )
+  install(FILES
+    ${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}.pc
+    DESTINATION ${TAOCPP_JSON_INSTALL_PKGCONFIG_DIR}
+  )
   install(EXPORT ${PROJECT_NAME}-targets
     NAMESPACE taocpp::
     DESTINATION ${TAOCPP_JSON_INSTALL_CMAKE_DIR}