folly: fix paths in .pc file

pkgs/development/libraries/folly/default.nix

@@ -59,6 +59,12 @@ stdenv.mkDerivation rec {
   NIX_CFLAGS_COMPILE = [ "-DFOLLY_MOBILE=${if follyMobile then "1" else "0"}" "-fpermissive" ];
   cmakeFlags = [ "-DBUILD_SHARED_LIBS=ON" ];
 
+  postFixup = ''
+    substituteInPlace "$out"/lib/pkgconfig/libfolly.pc \
+      --replace '=''${prefix}//' '=/' \
+      --replace '=''${exec_prefix}//' '=/'
+  '';
+
   # folly-config.cmake, will `find_package` these, thus there should be
   # a way to ensure abi compatibility.
   passthru = {