Define _POSIX_C_SOURCE globally

Stop trying to maintain a per-file _POSIX_C_SOURCE. Instead,
require POSIX.1-2008 globally. A lot of core source files depend
on that already.

Some care must be taken on a few select files where we need a bit
more than POSIX. Some files need XSI extensions (_XOPEN_SOURCE) and
some files need BSD extensions (_DEFAULT_SOURCE). In both cases,
these feature test macros imply _POSIX_C_SOURCE. Make sure to not
define both these macros and _POSIX_C_SOURCE explicitly to avoid
POSIX requirement conflicts (e.g. _POSIX_C_SOURCE says POSIX.1-2001
but _XOPEN_SOURCE says POSIX.1-2008).

Additionally, there is one special case in render/vulkan/vulkan.c.
That file needs major()/minor(), and these are system-specific.
On FreeBSD, _POSIX_C_SOURCE hides system-specific symbols so we need
to make sure it's not defined for this file. On Linux, we can
explicitly include <sys/sysmacros.h> and ensure that apart from
symbols defined there the file only uses POSIX toys.

render/gles2/pass.c

@@ -1,4 +1,3 @@
-#define _POSIX_C_SOURCE 199309L
 #include <stdlib.h>
 #include <assert.h>
 #include <pixman.h>

render/gles2/pixel_format.c

@@ -1,4 +1,3 @@
-#define _POSIX_C_SOURCE 199309L
 #include <drm_fourcc.h>
 #include <GLES2/gl2.h>
 #include <GLES2/gl2ext.h>

render/gles2/renderer.c

@@ -1,4 +1,3 @@
-#define _POSIX_C_SOURCE 199309L
 #include <assert.h>
 #include <drm_fourcc.h>
 #include <GLES2/gl2.h>

render/gles2/texture.c

@@ -1,4 +1,3 @@
-#define _POSIX_C_SOURCE 199309L
 #include <assert.h>
 #include <drm_fourcc.h>
 #include <GLES2/gl2.h>