build(CMake): Fix pkg-config for absolute CMAKE_INSTALL_*DIR

CMAKE_INSTALL_*DIR can be absolute, and in that case ${prefix} should
not be prepended.

See https://github.com/jtojnar/cmake-snips/?tab=readme-ov-file#concatenating-paths-when-building-pkg-config-files

c/CMakeLists.txt

@@ -1,4 +1,5 @@
-cmake_minimum_required(VERSION 3.9 FATAL_ERROR)
+# For cmake_path function
+cmake_minimum_required(VERSION 3.20 FATAL_ERROR)
 
 # respect C_EXTENSIONS OFF without explicitly setting C_STANDARD
 if (POLICY CMP0128)
@@ -229,6 +230,8 @@ install(FILES
     DESTINATION "${CMAKE_INSTALL_LIBDIR}/cmake/blake3"
 )
 
+cmake_path(APPEND libdir_for_pc_file "\${prefix}" "${CMAKE_INSTALL_LIBDIR}")
+cmake_path(APPEND includedir_for_pc_file "\${prefix}" "${CMAKE_INSTALL_INCLUDEDIR}")
 configure_file(libblake3.pc.in libblake3.pc @ONLY)
 install(FILES "${CMAKE_BINARY_DIR}/libblake3.pc"
   DESTINATION "${CMAKE_INSTALL_LIBDIR}/pkgconfig")

c/libblake3.pc.in

@@ -1,7 +1,7 @@
 prefix="@CMAKE_INSTALL_PREFIX@"
 exec_prefix="${prefix}"
-libdir="${prefix}/@CMAKE_INSTALL_LIBDIR@"
-includedir="${prefix}/@CMAKE_INSTALL_INCLUDEDIR@"
+libdir="@libdir_for_pc_file@"
+includedir="@includedir_for_pc_file@"
 
 Name: @PROJECT_NAME@
 Description: @PROJECT_DESCRIPTION@