[tlbuild] Building on El Capitan, Macintosh

[Richard Koch]

Folks,

Before leaving for a brief trip, I managed to compare compiling on El Capitan, which gives an errork, with compiling on High Sierra, which works. In both cases, the location

	source/Work/libs/icu/include/unicode/umachine.h

is ultimately referenced. These files are identical on the two systems. 

A central portion of this file is discussion of the type UChar. This is defined as a 16-bit integer. The file says "Starting with ICU 59, C++ API uses chalr16_t directly, while C API continues to use UChar."

Later it says "C++11 defines char16_t as bit-compatible with uint16_t, but as a distinct type." Then it says "On macOS, char16_t is not available because the uchar.h standard header is missing."

Then several other comments are made, which I haven't yet deciphered. Surely the El Capitan problem is created, and or explained, somewhere in this file.

I don't yet know a fix, which is in any case not needed on High Sierra.

Dick Koch