gegl-devel @0.4.50: builds fail on 10.13/10.14: dispatch.h: error: expected value in expression (HAVE_UNISTD_H)

[mascguy (Christopher Nielsen)]

Xcode Clang 9.x and 10.x fails on the following, for 10.13/10.14:

gegl-init.c.o -c ../gegl-0.4.50/gegl/gegl-init.c
In file included from ../gegl-0.4.50/gegl/gegl-init.c:59:
In file included from /System/Library/Frameworks/Foundation.framework/Headers/Foundation.h:6:
In file included from /opt/local/include/LegacySupport/CoreFoundation/CoreFoundation.h:27:
In file included from /System/Library/Frameworks/CoreFoundation.framework/Headers/CoreFoundation.h:60:
In file included from /System/Library/Frameworks/CoreFoundation.framework/Headers/CFPropertyList.h:17:
In file included from /System/Library/Frameworks/CoreFoundation.framework/Headers/CFStream.h:20:
/usr/include/dispatch/dispatch.h:38:45: error: expected value in expression
#if !defined(HAVE_UNISTD_H) || HAVE_UNISTD_H
                                            ^

Via the following new import in gegl-init.c, added with release 0.4.50:

#ifdef __APPLE__
#import <Foundation/Foundation.h>
#endif

Thoughts as to how we should deal with this, apart from blacklisting Clang 9.x/10.x (which seems absurd)?