libstdc++: Fix unwanted #pragma messages from PSTL headers [PR113376]

libstdc++: Fix unwanted #pragma messages from PSTL headers [PR113376]

When we rebased the PSTL on upstream, in r14-2109-g3162ca09dbdc2e, a
change to how _PSTL_USAGE_WARNINGS is set was missed out, but the change
to how it's tested was included. This means that the macro is always
defined, so testing it with #ifdef (instead of using #if to test its
value) doesn't work as intended.

Revert the test to use #if again, since that part of the upstream change
was unnecessary in the first place (the macro is always defined, so
there's no need to use #ifdef to avoid -Wundef warnings).

libstdc++-v3/ChangeLog:

	PR libstdc++/113376
	* include/pstl/pstl_config.h: Use #if instead of #ifdef to test
	the _PSTL_USAGE_WARNINGS macro.