On legacy systems, posix_spawnp() might return 0 even if the execvp()
invocation in the child had failed with ENOENT.
Replace posix_spawnp() with an internal, fork+execvp based internal
function.
* for a --target argument that precedes the compiler argument, as in:
slibtool --target=xxx --mode=compile cc
--> set an explicit (cross-)target,
--> but do not pass the argument to the compiler.
* for a --target argument that follows the compiler argument, as in:
slibtool --mode=compile cc --target=xxx
--> set an explicit (cross-)target,
--> and also pass the argument to the compiler.
