Merge #1692: cmake: configure libsecp256k1.pc during install

7106dce cmake: configure libsecp256k1.pc during install (Daniel Pfeifer)

Pull request description:

  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.

ACKs for top commit:
  real-or-random:
    ACK 7106dce I verified that it fixes the path in libsecp256k1.pc

Tree-SHA512: 34841513d2dc52234718eab56ecb9224aa1e13ad2d13cd103624b355e0627c37441363ad24293e07da7a748191e6ed2b67649b489bf874bab35346146b78c16f

Author: real-or-random

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()