Bug: https://bugs.gentoo.org/647422
Upstream Patch: https://sourceforge.net/p/freeimage/patches/130/

diff -Naur a/Source/FreeImage.h b/Source/FreeImage.h
--- a/Source/FreeImage.h	2018-03-24 23:24:11.066695395 -0400
+++ b/Source/FreeImage.h	2018-03-24 23:26:35.945391182 -0400
@@ -117,12 +117,17 @@
 #ifndef _WINDOWS_
 #define _WINDOWS_
 
+#if JPEG_LIB_VERSION < 90
+
 #ifndef FALSE
 #define FALSE 0
 #endif
 #ifndef TRUE
 #define TRUE 1
 #endif
+
+#endif
+
 #ifndef NULL
 #define NULL 0
 #endif