[flang] Fix Darwin build after 4762c65

Select POSIX 2008 standard to avoid including Darwin extensions.
Otherwise, Darwin's math.h header defines HUGE, which conflicts
with Flang's HUGE function.

This is a temporary build fix. More permanent solutions would be to
rename Flang's HUGE or to define _POSIX_C_SOURCE for Flang as a
whole.

This started happening after 4762c65 (llvm#82443), that added the
"utility" include, which seems to include "math.h".

Author: luporl

flang/lib/Decimal/decimal-to-binary.cpp

@@ -6,6 +6,8 @@
 //
 //===----------------------------------------------------------------------===//
 
+#define _POSIX_C_SOURCE 200809
+
 #include "big-radix-floating-point.h"
 #include "flang/Common/bit-population-count.h"
 #include "flang/Common/leading-zero-bit-count.h"