Log Message:
Tokenize the contents of "append_executable"

This configuration parameter is used when linking executables, through
cc(1) or ld(1) for instance.

This allows passing multiple arguments via the "append_executable"
option, like "-fPIC -pie" for instance.

Committing this before joerg@'s approval since a call for testing
PKGSRC_MKPIE has just been issued, and this helps with some packages.