math/why3: Unbreak after ocaml-findlib change

Use the same technique madpilot used on x11-toolkits/ocaml-lablgtk2
to restore the build after the (unexpected) changed to the output
of ocamlfindlib during its update to 1.7.1

While here, document previously unknown ocamlfind requirement.

math/why3/Makefile

@@ -27,6 +27,7 @@ INSTALL_TARGET=	install-all
 
 USES=		gmake
 USE_OCAML=	yes
+USE_OCAML_FINDLIB=	yes
 
 FEDORA=		${DISTNAME}${EXTRACT_SUFX}/35f99e5f64939e50ea57f641ba2073ec
 ALL_TARGET=	all byte

math/why3/files/patch-configure

@@ -0,0 +1,11 @@
+--- configure.orig	2014-03-14 15:01:05 UTC
++++ configure
+@@ -3427,7 +3427,7 @@ fi
+ 
+ #if ocamlfind is used it gives the install path for ocaml library
+ if test "$USEOCAMLFIND" = yes; then
+-   OCAMLINSTALLLIB=$(ocamlfind printconf destdir)
++   OCAMLINSTALLLIB=$(ocamlfind printconf destdir | sed 's/:.*//')
+ else
+    OCAMLINSTALLLIB=$OCAMLLIB
+ fi