restrict to linux only due to `yes no|omake` spinning on non-Linux
Switched to OCamlMakefile.
Use netstring-pcre package.

In ocamlnet-3.6.5, Netstring_pcre module is packaged separately
in netstring-pcre.
See https://godirepo.camlcity.org/wwwsvn/tags/ocamlnet-3.6.5/ChangeLog?rev=1858&root=lib-ocamlnet2&view=markup
Merge branch 'master' of github.com:mirage/opam-repository
+cow.0.6.1
Merge pull request #873 from mirage/master

cow.0.6.1
ocp-indent 1.3.0
Add riakc 2.0.0, has minimal 2i support
Merge pull request #874 from orbitz/add-riakc-2.0.0

Add riakc 2.0.0, has minimal 2i support
Merge pull request #859 from vbmithr/master

New version of ocaml-crypt.
Merge pull request #872 from scjung/fix-ocsigenserver

Use netstring-pcre package to build ocsigenserver.