Fix hscollider build on alpine linux

[smlx]

_exit() is defined in <unistd.h> as per POSIX.

I don't have access to a Windows box so can't test the build on that platform.

[carenas]

while the proposed code does "fix" the build it doesn't address the root cause (which was that the header needed for _exit() was being included through the wrong dependency, and the test for it was never used), moving instead the dependency to another flag that also happens to work for alpine linux

and alternative solution is to install libexecinfo-dev in alpine so the logic that was originally added for FreeBSD can do its magic, instead