cmake: configure libsecp256k1.pc during install

When installing to a given prefix, make sure that the .pc file contains
that prefix rather than the value of CMAKE_INSTALL_PREFIX that the
project was configured with.

Author: purpleKarrot

cmake/GeneratePkgConfigFile.cmake

@@ -1,8 +1,12 @@
 function(generate_pkg_config_file in_file)
-  set(prefix ${CMAKE_INSTALL_PREFIX})
+  set(prefix "@CMAKE_INSTALL_PREFIX@")
   set(exec_prefix \${prefix})
   set(libdir \${exec_prefix}/${CMAKE_INSTALL_LIBDIR})
   set(includedir \${prefix}/${CMAKE_INSTALL_INCLUDEDIR})
   set(PACKAGE_VERSION ${PROJECT_VERSION})
-  configure_file(${in_file} ${PROJECT_NAME}.pc @ONLY)
+  configure_file(${in_file} ${PROJECT_NAME}.pc.in @ONLY)
+  install(CODE "configure_file(
+  \"${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}.pc.in\"
+  \"${CMAKE_CURRENT_BINARY_DIR}/${PROJECT_NAME}.pc\"
+  @ONLY)" ALL_COMPONENTS)
 endfunction()