disable fork-malloc-exec for non-glibc-systems

Calling malloc() after fork is undefined behaviour if the process is
multi-threaded. locks held by a thread on fork() will never be released.
malloc() is usally protected by a lock and can therefore deadlock. glibc
is known not to deadlock in this case.

This commit does not rule out other problems on glibc-systems, but fixes an
issue on musl-libc-systems. Only restricting to async-signal safe functions
between fork() and exec() prevents undefined behaviour for sure. See
signal-safety(7).
11 jobs for glibc in 9 minutes and 47 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #4714497
build:cmake

00:03:51

passed #4714494
build:debug

00:08:46

manual #4714499
allowed to fail manual
build:i686-w64-mingw32-cmake
passed #4714498
build:i686-w64-mingw32-debug

00:04:46

manual #4714496
allowed to fail manual
build:legacy
passed #4714493
build:production

00:05:05

manual #4714495
allowed to fail manual
build:reduced
manual #4714502
allowed to fail manual
build:stretch
passed #4714500
build:x86_64-w64-mingw32

00:05:25

passed #4714501
build:x86_64-w64-mingw32-cmake-debug

00:09:44

manual #4714503
allowed to fail manual
build:xenial