With musl, mesh.cpp compilation fails as struct timeval is not defined
although <time.h> is included before "timer.h" which uses this data
type.

The reason is with musl contrary to glibc, <time.h> does not include
<sys/time.h> (which provides gettimeofday, struct timeval).

Fixes llvm/llvm-project#94320

musl implements random() differently than glibc.

The output of the program is thus dependent on this implementation
detail and could differ between platforms. To remediate that, we
replace it by a very simple PRNG (ANSI C LCG).

Warning: [^1] indicates this PRNG have quite a poor LSB randomness.

[^1]: Entacher, Karl. (1999). A Collection of Selected Pseudorandom
Number Generators With Linear Structures.

Fixes llvm/llvm-project#94048

Compatible with musl in particular. <unistd.h> is already included in
"config.h".

It supposes the system does not "lie" by providing only a partial POSIX
support.

FreeBSD/NetBSD/OpenBSD/Apple (XNU) provides the macro too but are left
untouched as I don't have the means to test it.

Duplicate of an abandoned patch from 2015: https://reviews.llvm.org/D13938