Makefile for Linux Mint, improved OS detection, MIME-types of POST...

[jlerouge]

• I've added some routines to auto-detect the Linux distribution, and choose the good Makefile. Cross-compilation should work as-is, but I can't test it for CentOS and SUSE. It's all good for Ubuntu and Linux Mint.
• I've fixed the httpd/request.js source, in order to accept all character-based MIME types in POST data. I needed it in order to send JSON by POST.
• I've modified the QUICKSTART