compat: wrap math.h to avoid AIX-specific clashes

AIX defines a class() function in its math.h header without any
guard.

compat/aix/math.h

+/*
+ * Workaround aix-specific class() function clashing with libav class usage
+ */
+
+#ifndef COMPAT_AIX_MATH_H
+#define COMPAT_AIX_MATH_H
+
+#define class class_in_math_h_causes_problems
+
+#include_next <math.h>
+
+#undef class
+
+#endif /* COMPAT_AIX_MATH_H */

configure

@@ -3049,6 +3049,9 @@ enabled spic && enable_weak pic
 
 # OS specific
 case $target_os in
+    aix)
+        add_cppflags '-I\$(SRC_PATH)/compat/aix'
+        ;;
     haiku)
         prefix_default="/boot/common"
         network_extralibs="-lnetwork"