Require OCaml 4.04 or later

Earlier OCaml versions do not reliably support our use of the "%identity"
primitive.

README.md

@@ -22,15 +22,14 @@ on large intergers use the low-level MPN functions from GMP.
 
 Arbitrary-precision integers can be compared correctly using OCaml's
 polymorphic comparison operators (`=`, `<`, `>`, etc.).
-This requires OCaml version 3.12.1 or later, though.
 
 Additional features include:
 * a module `Q` for rationals, built on top of `Z` (see `q.mli`)
 * a compatibility layer `Big_int_Z` that implements the same API as Big_int from the legacy `Num` library, but uses `Z` internally
 
 ## REQUIREMENTS
 
-* OCaml, preferably version 3.12.1 or later.  (Earlier versions are usable but generic comparisons will misbehave.)
+* OCaml, version 4.04.0 or later.
 * Either the GMP library or the MPIR library, including development files.
 * GCC or Clang or a gcc-compatible C compiler and assembler (other compilers may work).
 * The Perl programming language.

configure

@@ -396,34 +396,27 @@ if test "$gmp" != 'OK'; then echo "cannot find GMP nor MPIR"; exit 2; fi
 
 ocamlver=`ocamlc -version`
 
-# Extended comparisons available since 3.12.1
+# OCaml version 4.04 or later is required
+
 case "$ocamlver" in
-    [12].* | 3.0* | 3.10* | 3.11* | 3.12.0*)
-        ;;
-    *)
-        echo "OCaml extended comparison supported"
-        ccdef="-DZ_OCAML_COMPARE_EXT $ccdef"
+    [123].* | 4.0[0123].*)
+        echo "OCaml version $ocamlver is no longer supported."
+        echo "OCaml version 4.04.0 or later is required."
+        exit 2
         ;;
 esac
 
+# Extended comparisons available since 3.12.1
+echo "OCaml extended comparison supported"
+ccdef="-DZ_OCAML_COMPARE_EXT $ccdef"
+
 # New hash functions available since 4.00.0
-case "$ocamlver" in
-    [123].*)
-        ;;
-    *)
-        echo "OCaml new hash functions available"
-        ccdef="-DZ_OCAML_HASH $ccdef"
-        ;;
-esac
+echo "OCaml new hash functions available"
+ccdef="-DZ_OCAML_HASH $ccdef"
 
 # -bin-annot available since 4.00.0
-case "$ocamlver" in
-    [123].*)
-        hasbinannot='no';;
-    *)
-        echo "OCaml supports -bin-annot to produce documentation"
-        hasbinannot='yes';;
-esac
+echo "OCaml supports -bin-annot to produce documentation"
+hasbinannot='yes'
 
 # Changes to C API (the custom_operation struct) since 4.08.0
 case "$ocamlver" in