[flang] Fix Darwin build after 4762c6557d15 (#84478)
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 started happening after 4762c6557d15 (#82443), that added the
"utility" include, which seems to include "math.h".
diff --git a/flang/CMakeLists.txt b/flang/CMakeLists.txt
index 21617ae..71141e5 100644
--- a/flang/CMakeLists.txt
+++ b/flang/CMakeLists.txt
@@ -413,6 +413,14 @@
endif()
+# Clang on Darwin enables non-POSIX extensions by default, which allows the
+# macro HUGE to leak out of <math.h> even when it is never directly included,
+# conflicting with Flang's HUGE symbols.
+# Set _POSIX_C_SOURCE to avoid including these extensions.
+if (APPLE)
+ set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -D_POSIX_C_SOURCE=200809")
+endif()
+
list(REMOVE_DUPLICATES CMAKE_CXX_FLAGS)
# Determine HOST_LINK_VERSION on Darwin.
