Generate pkg-config files under CMake

configure_pkg_config_file helper function copied from the ogg
repository.

CMakeLists.txt

@@ -83,6 +83,21 @@ target_compile_definitions(opusfile
     $<$<BOOL:${OP_ENABLE_ASSERTIONS}>:OP_ENABLE_ASSERTIONS>
     $<$<BOOL:${OP_HAVE_LRINTF}>:OP_HAVE_LRINTF>
 )
+
+# Helper function to configure pkg-config files
+function(configure_pkg_config_file pkg_config_file_in)
+    set(prefix ${CMAKE_INSTALL_PREFIX})
+    set(exec_prefix ${CMAKE_INSTALL_FULL_BINDIR})
+    set(libdir ${CMAKE_INSTALL_FULL_LIBDIR})
+    set(includedir ${CMAKE_INSTALL_FULL_INCLUDEDIR})
+    set(VERSION ${PROJECT_VERSION})
+    string(REPLACE ".in" "" pkg_config_file ${pkg_config_file_in})
+    configure_file(${pkg_config_file_in} ${pkg_config_file} @ONLY)
+endfunction()
+
+configure_pkg_config_file(opusfile.pc.in)
+configure_pkg_config_file(opusurl.pc.in)
+
 install(TARGETS opusfile
   EXPORT OpusFileTargets
   RUNTIME DESTINATION "${CMAKE_INSTALL_BINDIR}"
@@ -92,6 +107,10 @@ install(TARGETS opusfile
   PUBLIC_HEADER DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}/opus"
 )
 
+install(FILES ${CMAKE_CURRENT_BINARY_DIR}/opusfile.pc ${CMAKE_CURRENT_BINARY_DIR}/opusurl.pc
+  DESTINATION ${CMAKE_INSTALL_LIBDIR}/pkgconfig
+)
+
 if(NOT OP_DISABLE_HTTP)
   find_package(OpenSSL REQUIRED)