cmake: Install libsecp256k1.pc file

This change allows downstream projects to use pkg-config to search for
the libsecp256k1 library that is built with CMake.

Author: hebasto

cmake/GeneratePkgConfigFile.cmake

@@ -0,0 +1,8 @@
+function(generate_pkg_config_file in_file)
+  set(prefix ${CMAKE_INSTALL_PREFIX})
+  set(exec_prefix \${prefix})
+  set(libdir \${exec_prefix}/${CMAKE_INSTALL_LIBDIR})
+  set(includedir \${prefix}/${CMAKE_INSTALL_INCLUDEDIR})
+  set(PACKAGE_VERSION ${PROJECT_VERSION})
+  configure_file(${in_file} ${PROJECT_NAME}.pc @ONLY)
+endfunction()

src/CMakeLists.txt

@@ -161,5 +161,13 @@ if(SECP256K1_INSTALL)
       ${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}-config.cmake
       ${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}-config-version.cmake
     DESTINATION ${CMAKE_INSTALL_LIBDIR}/cmake/${PROJECT_NAME}
-)
+  )
+
+  include(GeneratePkgConfigFile)
+  generate_pkg_config_file(${PROJECT_SOURCE_DIR}/libsecp256k1.pc.in)
+  install(
+    FILES
+      ${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}.pc
+    DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig
+  )
 endif()