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-rw-r--r--src/mesa/main/imports.h2
1 files changed, 1 insertions, 1 deletions
diff --git a/src/mesa/main/imports.h b/src/mesa/main/imports.h
index a439370bc4c..e967b147311 100644
--- a/src/mesa/main/imports.h
+++ b/src/mesa/main/imports.h
@@ -120,7 +120,7 @@ typedef union { GLfloat f; GLint i; } fi_type;
* \name Work-arounds for platforms that lack C99 math functions
*/
/*@{*/
-#if (_XOPEN_SOURCE < 600) && !defined(_ISOC99_SOURCE) \
+#if (defined(_XOPEN_SOURCE) && (_XOPEN_SOURCE < 600)) && !defined(_ISOC99_SOURCE) \
&& (!defined(__STDC_VERSION__) || (__STDC_VERSION__ < 199901L)) \
&& (!defined(_MSC_VER) || (_MSC_VER < 1400))
#define acosf(f) ((float) acos(f))