http://bugs.gentoo.org/520906

This kludge is unnecessary with jpeg-9a and is in fact breaking the build...

libmng_jpeg.c:108:10: error: 'FALSE' undeclared (first use in this function)
libmng_jpeg.c:536:48: error: 'TRUE' undeclared (first use in this function)
libmng_jpeg_c:890:49: error: 'TRUE' undeclared (first use in this function)

...because jmorecfg.h of jpeg-9a has this:

#ifndef HAVE_BOOLEAN
#if defined FALSE || defined TRUE || defined QGLOBAL_H
/* Qt3 defines FALSE and TRUE as "const" variables in qglobal.h */
typedef int boolean;
#ifndef FALSE                   /* in case these macros already exist */
#define FALSE   0               /* values of boolean */
#endif
#ifndef TRUE
#define TRUE    1
#endif
#else
typedef enum { FALSE = 0, TRUE = 1 } boolean;
#endif
#endif

2018-09-06: Adjusted for libmng-2.0.3
https://bugs.gentoo.org/578744

--- a/libmng_types.h	2015-04-20 21:17:32.000000000 +0200
+++ b/libmng_types.h	2018-09-06 22:56:57.153190623 +0200
@@ -200,13 +200,6 @@
 #undef FAR                             /* possibly defined by zlib or lcms */
 #endif
 #define JPEG_INTERNAL_OPTIONS          /* for RGB_PIXELSIZE */
-/* There has been a change in jpeg-9 : */
-#if !defined(HAVE_BOOLEAN) && !defined(_WIN32)
-#define HAVE_BOOLEAN
-#endif
-#ifndef _WIN32
-typedef int boolean;
-#endif
 /* For jpegsrc.v09a: */
 #include <stdio.h>
 #include <jpeglib.h>