ocaml-gsl: realign discover.ml patch so it applies on older macOS

Signed-off-by: Paul Guyot <pguyot@kallisys.net>

math/ocaml-gsl/files/patch-discover-gsl-include.diff

@@ -1,8 +1,6 @@
 --- src/config/discover.ml.orig
 +++ src/config/discover.ml
-@@ -18,7 +18,8 @@
-       let conf =
-         let default =
+@@ -26,7 +26,8 @@
            { libs = [ "-lgsl"; "-lgslcblas"; "-lm" ]; cflags = [] }
          in
          let write_gsl_include = C.Flags.write_lines "gsl_include.sexp" in
@@ -10,3 +8,5 @@
 +        let default_gsl_include =
 +          [ (try Sys.getenv "GSL_INCLUDE_DIR" with Not_found -> "/usr/include") ] in
          match C.Pkg_config.get c with
+         | None ->
+             write_gsl_include default_gsl_include;