First of all: C89 or better.  If you don't have that, port gcc first.

Use of C language extensions throughout the X server tree
---------------------------------------------------------

Optional extensions:
The server will still build if your toolchain does not support these
extensions, although the results may not be optimal.

    * _X_SENTINEL(x): member x of the passed structure must be NULL, e.g.:
                      void parseOptions(Option *options _X_SENTINEL(0));
                      parseOptions("foo", "bar", NULL); /* this is OK */
                      parseOptions("foo", "bar", "baz"); /* this is not */
                      This definition comes from Xfuncproto.h in the core
                      protocol headers.
    * _X_ATTRIBUTE_PRINTF(x, y): This function has printf-like semantics;
                                 check the format string when built with
                                 -Wformat (gcc) or similar.
    * _X_EXPORT: this function should appear in symbol tables.
    * _X_HIDDEN: this function should not appear in the _dynamic_ symbol
                 table.
    * _X_INTERNAL: like _X_HIDDEN, but attempt to ensure that this function
                   is never called from another module.
    * _X_INLINE: inline this functon if possible (generally obeyed unless
                 disabling optimisations).
    * _X_DEPRECATED: warn on use of this function.

Mandatory extensions:
The server will not build if your toolchain does not support these extensions.

    * named initialisers: explicitly initialising structure members, e.g.:
                          struct foo bar = { .baz = quux, .brian = "dog" };
    * variadic macros: macros with a variable number of arguments, e.g.:
                       #define DebugF(x, ...) /**/
    * interleaved code and declarations: { foo = TRUE; int bar; do_stuff(); }


Use of OS and library facilities throughout the X server tree
-------------------------------------------------------------

Non-OS-dependent code can assume facilities at least as good as
the non-OS-facility parts of POSIX-1.2001.  Ideally this would
be C99, but even gcc+glibc doesn't implement that yet.

Unix-like systems are assumed to be at least as good as UNIX03.

Linux systems must be at least 2.4 or later.  As a practical matter
though, 2.4 kernels never receive any testing.  Use 2.6 already.

TODO: Solaris.

TODO: *BSD.

Code that needs to be portable to Windows should be careful to,
well, be portable.  Note that there are two Windows ports, cygwin and
mingw.  Cygwin is more or less like Linux, but mingw is a bit more
restrictive.  TODO: document which versions of Windows we actually care
about.

OSX support is generally limited to the most recent version.  Currently
that means 10.5.