Comment # 1 on bug 1179945 from Dominique Leuenberger

From the ICU 68 website:

Migration Issues

New in ICU 68

    ICU4C public header files no longer define and use the macros FALSE and
TRUE. (ICU-21267)
        This avoids collisions between these macro definitions and application
code that defines enum constants with these names.
        The ICU macros are no longer necessary: C++11 and C99 define false &
true values.
        ICU API continues to use the ICU-specific type UBool for now; the
standard values can be assigned to UBool variables and arguments without
warnings.
        Please change call sites to use the standard false & true values where
necessary. For C (as opposed to C++), these are also macros, defined in
<stdbool.h>.
        You may transitionally define U_DEFINE_FALSE_AND_TRUE=1 if you need
time to migrate code, for example in application code before including any ICU
header file, or by patching unicode/umachine.h and changing #   define
U_DEFINE_FALSE_AND_TRUE 0 to assign value 1 instead.