This change is only useful for the conformance tests since the headers
changed are not installed. The conformance tests fail due to IS_IN
not being defined, so wrap it with a check to make sure that _ISOMAC
is defined.
* include/bits/stdlib-float.h: Use IS_IN only if _ISOMAC is
defined.
* include/mqueue.h: Likewise.
* include/stdlib.h: Likewise.