Build fixes for OpenBSD

[pstumpf]

Just some ifdef glue to make it build on OpenBSD.

diff --git a/export/OpenColorIO/OpenColorABI.h.in b/export/OpenColorIO/OpenColorABI.h.in0a3fb..f1dfa67 100644
--- a/export/OpenColorIO/OpenColorABI.h.in
AMAGE.

 // If supported, define OCIOEXPORT, OCIOHIDDEN
 // (used to choose which symbols to export from OpenColorIO)
-#if defined __linux__ || __APPLE__ || __FreeBSD__
+#if defined __linux__ || __APPLE__ || __FreeBSD__ || __OpenBSD__
     #if __GNUC__ >= 4
         #define OCIOEXPORT __attribute__ ((visibility("default")))
         #define OCIOHIDDEN __attribute__ ((visibility("hidden")))
diff --git a/src/core/Platform.h b/src/core/Platform.h
index d2d3f30..320e3ed 100644
--- a/src/core/Platform.h
+++ b/src/core/Platform.h
@@ -85,7 +85,7 @@ OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGES.

 // linux/unix/posix
 #include <stdlib.h>
-#if !defined(__FreeBSD__)
+#if !defined(__FreeBSD__) && !defined(__OpenBSD__)
 #include <alloca.h>
 #endif
 #include <string.h>