pkgs/build-support/cabal: fix spelling of the --extra-{include,lib}-dirs options to Cabal

It's quite amazing that we've managed to pass incorrectly spelled command line
flags to Cabal for ages without ever noticing. :-)

The search path options --extra-{include,lib}-dirs are usually unnecessary,
because the build environment is set up such that gcc and ld find those headers
and libraries automatically, i.e. without needing extra flags. The bubble burst
on MacOS X, though, where the build of haskell-text-icu couldn't find the icu
library without manually setting DYLD_LIBRARY_PATH in that build. Fortunately,
cabal takes care of that issue if a correctly spelled --extra-lib-dirs flag is
passed.

pkgs/build-support/cabal/default.nix

@@ -115,11 +115,11 @@
                   continue;
                 fi
                 if [ -d "$p/include" ]; then
-                  extraConfigureFlags+=" --extra-include-dir=$p/include"
+                  extraConfigureFlags+=" --extra-include-dirs=$p/include"
                 fi
                 for d in lib{,64}; do
                   if [ -d "$p/$d" ]; then
-                    extraConfigureFlags+=" --extra-lib-dir=$p/$d"
+                    extraConfigureFlags+=" --extra-lib-dirs=$p/$d"
                   fi
                 done
               done