fixup! git-compat-util: avoid redeclaring _DEFAULT_SOURCE

Part of the series that drops the vendored `mimalloc` from this fork;
the rationale (no measurable speedup over the platform allocator on
any of Windows, macOS, or Linux) is in the second commit of the
series and the PR description. The `fixup!` subject is so the next
rebase against an upstream Git that already lacks the target commit
autosquashes this revert into it, dropping the original cleanly.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Author: dscho

compat/posix.h

@@ -81,9 +81,7 @@
 #define _ALL_SOURCE 1
 #define _GNU_SOURCE 1
 #define _BSD_SOURCE 1
-#ifndef _DEFAULT_SOURCE
 #define _DEFAULT_SOURCE 1
-#endif
 #define _NETBSD_SOURCE 1
 #define _SGI_SOURCE 1