Because of some overlapping changes I happened to stumble on a problem
with a macro definition introduced in fixing https://bugs.openjdk.java.net/browse/JDK-8164818

// Use 16.16 for better precision than 26.6
#define FloatToFixedScale ((float)(1 << 16))
#define FloatToFixed(f) ((unsigned int)((f) * FloatToFixedScale))

I found that if I changed "16" to anything else my change was ignored on Mac.

The reason is that a definition in an MacOS framework header which just
so happened to amount to the same as I was using was always winning.
/System/Library/Frameworks/CoreServices.framework/Frameworks/CarbonCore.framework/Headers/FixMath.h has
#define fixed1 ((Fixed) 0x00010000L)
#define FloatToFixed(a) (_IntSaturate((a) * fixed1))

So why was there no redefinition warning ?
The order of includes I have is :
#include "hb.h"
#include "hb-jdk.h"
#ifdef MACOSX
#include "hb-coretext.h"
#endif
#include <stdlib.h>

If I swap this to
#ifdef MACOSX
#include "hb-coretext.h"
#endif
#include "hb.h"
#include "hb-jdk.h"
#include <stdlib.h>

Then I get a warning :

Building target 'default (exploded-image)' in configuration 'macosx-x86_64-normal-server-release'
Building JVM variant 'server' with features 'all-gcs cds closed-src commercial-features compiler1 compiler2 dtrace fprof jni-check jvmci jvmti management nmt services trace vm-structs'
Updating libfontmanager.dylib due to 1 file(s)
In file included from ~/jdk9-client/jdk/src/java.desktop/share/native/libfontmanager/hb-jdk-font.cc:30:
~/jdk9-client/jdk/src/java.desktop/share/native/libfontmanager/hb-jdk.h:56:9: error: 'FloatToFixed' macro redefined [-Werror,-Wmacro-redefined]
#define FloatToFixed(f) ((unsigned int)((f) * 64.0f))
        ^
/System/Library/Frameworks/CoreServices.framework/Frameworks/CarbonCore.framework/Headers/FixMath.h:93:9: note: previous definition is here
#define FloatToFixed(a) (_IntSaturate((a) * fixed1))
        ^
1 error generated.
make[3]: *** [~/jdk9-client/build/macosx-x86_64-normal-server-release/support/native/java.desktop/libfontmanager/hb-jdk-font.o] Error 1
make[2]: *** [java.desktop-libs] Error 2

I do not know why "no warning" in the case when apparently the Carbon definition over-rides mine but not vice versa.
I can't find an undef or a pragma that would cause this.
But to make guard against this I intend to make my macro name less generic.