gles3.20160307 is not compatible with ocaml-5
```
=== ERROR while compiling gles3.20160307.alpha ===============================#
context 2.2.0~alpha~dev | linux/x86_64 | ocaml-base-compiler.5.0.0 | file:///home/opam/opam-repository
path ~/.opam/5.0/.opam-switch/build/gles3.20160307.alpha
command ~/.opam/opam-init/hooks/sandbox.sh build make
exit-code 2
env-file ~/.opam/log/gles3-7-3241c2.env
output-file ~/.opam/log/gles3-7-3241c2.out
ocamlfind ocamldep egl.ml egl.mli > egl.dep
ocamlfind ocamldep textures.ml textures.mli > textures.dep
ocamlfind ocamldep buffers.ml buffers.mli > buffers.dep
ocamlfind ocamldep shaders.ml shaders.mli > shaders.dep
ocamlfind ocamldep matrix.ml matrix.mli > matrix.dep
ocamlfind ocamldep gles3.ml gles3.mli > gles3.dep
ocaml maketags.ml < gles3_tags.lst > gles3_tags.h
File "./maketags.ml", line 59, characters 10-26:
59 | "GL_" ^ String.uppercase tag
^^^^^^^^^^^^^^^^
Error: Unbound value String.uppercase
```
Signed-off-by: Marcello Seri <marcello.seri@gmail.com>