Fix configure.ac checking for USE_MD5SUM

'configure.ac' generates 'configure' with invalid syntax,
it is not allowed to test integer like this

Signed-off-by: Jan Havran <havran.jan@email.cz>

configure: update -mmacosx-version-min= to 10.6

This could be done other ways if it turns out there's a flood of
people out there desperate for 10.4 or 10.5 compatibility, but I
expect this is safe enough to solve in the simplest possible way,
which is by just bumping the number here to 10.6.

Fixes https://github.com/jackaudio/jack1/issues/81.

read/write wrappers to handle the case of interruption by system signals

Atomics cleanup

Use atomic_fetch_add from C11's stdatomic if available or fallback to
GCC's __atomic functions.

Remove atomic_add because is can trivially be implemented by calling
exchange_and_add instead.

Using the process realtime priority instead of the thread realtime priority when sufficient.

config/cpu cleanup

Use LC_ALL=C to generate manpage date

ifdef is useless, since the value is always defined (either to 1 or 0) in config.h.

Fix inspired by

  https://svnweb.freebsd.org/ports/head/audio/jack/files/patch-drivers_dummy_dummy__driver.c?view=markup

Also simplified #if condition in libjack/time.c - it's easier to read a positive than a negated
condition.

ifdef is useless, since the value is always defined (either to 1 or 0) in config.h.

Fix taken from

  https://svnweb.freebsd.org/ports/head/audio/jack/files/patch-drivers_dummy_dummy__driver.c?view=markup

glibc above 2.17 has these functions builtin, so no need to link against librt.

When the macro succeeds, it needs to set the define to 1, not 0.

Also clarify the text message if librt is linked in.

Without this, the user's locale may influence the content of the
manpages (eg juin instead of June for a french locale).

Closes #54, thanks Rui!

Fixes negative values in xrun reports on Linux kernels 4.0 and above.

Update OS X CoreAudio AudioUnits API usage.

Moves VALGRIND_MEMSET above USE_CAPABILITIES ifdef

OpenBSD compilable :

allow to use source date for man page generation

Add support for travis

Check for clock_nanosleep before use

Sets memory for valgrind even if USE_CAPABILITIES is not defined.

Fixes a homebrew issue (https://github.com/Homebrew/legacy-homebrew/issues/43874)

Fixed leaked memory with jack_tmpdir.

Uses static local variable like how jack_user_dir does. jack_get_tmpdir()
replaces jack_tmpdir for the most part except in jackd -l where it would
cause an infinite loop, and instead DEFAULT_TMP_DIR is used, which is
what it would happen anyway. Also in jack_user_dir the default value is
used if jack_user_dir returns NULL from some error, but an error message
is printed with jack_error(), this is the same as the old behavior, but
I am not sure this is quite what should happen. Still, the exact same
behavior as before is acheived except without the use of the jack_tmpdir
global and the memory issues caused by calling jack_get_tmpdir().

Fixes a build failure on macOS 10.12 Sierra

- md5sum is not available.
- adding sndio support from Jacob Meuser <jakemsr@sdf.lonestar.org>.
- various patches from ajacoutot@, jasper@ and sthen@

this enables making reproducible builds of this package

This reverts commit 423931219d.