[gcc r15-1255] libstdc++: Fix unwanted #pragma messages from PSTL headers [PR113376]

[Jonathan Wakely]

https://gcc.gnu.org/g:99a1fe6c12c733fe4923a75a79d09a66ff8abcec

commit r15-1255-g99a1fe6c12c733fe4923a75a79d09a66ff8abcec
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Wed Jun 12 16:47:17 2024 +0100

    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.

Diff:
---
 libstdc++-v3/include/pstl/pstl_config.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/libstdc++-v3/include/pstl/pstl_config.h b/libstdc++-v3/include/pstl/pstl_config.h
index 7157a8a492ed..6fae15460eac 100644
--- a/libstdc++-v3/include/pstl/pstl_config.h
+++ b/libstdc++-v3/include/pstl/pstl_config.h
@@ -177,7 +177,7 @@
 
 #define _PSTL_PRAGMA_MESSAGE_IMPL(x) _PSTL_PRAGMA(message(_PSTL_STRING_CONCAT(_PSTL_PRAGMA_LOCATION, x)))
 
-#if defined(_PSTL_USAGE_WARNINGS)
+#if _PSTL_USAGE_WARNINGS
 #    define _PSTL_PRAGMA_MESSAGE(x) _PSTL_PRAGMA_MESSAGE_IMPL(x)
 #    define _PSTL_PRAGMA_MESSAGE_POLICIES(x) _PSTL_PRAGMA_MESSAGE_IMPL(x)
 #else