BSD headers: remove _BSD_SOURCE feature gate for sysexits.h

During the compilation of LLVM version 8, the build failed because it depends
on a constant in this file. In hrev49042 all BSD headers were feature-gated by
_BSD_SOURCE. This is not done (for this file) in glibc and (obviously) not in
BSD's libc.

Since this is not common practise, I would propose removing the feature gate
for this header file, as it would mean that we would have to upstream patches
for ports of other software that depends on the availability of these
constants.

Change-Id: I486f0c2e87eff489ce92d03589a6299ef1be6ca5
Reviewed-on: https://review.haiku-os.org/c/1144
Reviewed-by: waddlesplash <waddlesplash@gmail.com>

Protect all BSD extensions with _BSD_SOURCE

* Do not define the symbols by default, as they are not in the default
libraries.
* Adjust jamfiles of all code using BSD extensions to define
_BSD_SOURCE.
* This makes Haiku slightly more compliant to standard C/POSIX.

Add sysexits.h from FreeBSD r259958

I'd need this for getconf.

Protect all BSD extensions with _BSD_SOURCE

* Do not define the symbols by default, as they are not in the default
libraries.
* Adjust jamfiles of all code using BSD extensions to define
_BSD_SOURCE.
* This makes Haiku slightly more compliant to standard C/POSIX.

Add sysexits.h from FreeBSD r259958

I'd need this for getconf.