Fixes compiler warning.

This was used by proprietary software.  MPD adopted it a few years
ago, which turns out to be a mistake, because it now creates problems
for some MPD users (http://bugs.musicpd.org/view.php?id=4168).

Broken by commit cdbdcec7

Android releases were missing since 0.19.1.

Simplify the definition of many build options.

Fixes linker error due to missing "config_domain" symbol.

Don't assign bash variables; instead, use the m4 parameters directly.

A negative uid value means it's not a "local socket" (PF_LOCAL).
uid==0 means user "root" connected.

Must return -1 on error, not 0.  0 is root.

Check opus_decoder!=nullptr instead.

Commit d42c0f1d added an OS X-specific
method of calling mpd_main_after_fork(), which uses Grand Central
Dispatch. Since this uses a block literal, it breaks compilation on
compilers which don't support the block extension, e.g. non-Apple
compilers. This affects users on older OS X releases with GCD (which
depend on older Clang releases, or Apple GCCs, which don't support the
C++11 features MPD needs); or which don't support GCD at all (10.5 and
lower).

This patch changes the #ifdef so that the non-GCD code is used
as it was on OS X before this patch if blocks aren't available, via
checking __BLOCKS__ macro.