[tlbuild] upmendex doesn't compile

[Marc Baudoin]

Hi,

upmendex doesn't compile anymore (probably because of the last
commits) :

gcc -DHAVE_CONFIG_H -I. -I../../../texk/upmendex -I/usr/local/src/texlive/Work/texk -I/usr/local/src/texlive/texk -Wimplicit -Wreturn-type -Wdeclaration-after-statement -Wno-unknown-pragmas -g -O2 -MT convert.o -MD -MP -MF $depbase.Tpo -c -o convert.o ../../../texk/upmendex/convert.c &&\
mv -f $depbase.Tpo $depbase.Po
In file included from ../../../texk/upmendex/convert.c:1:0: ../../../texk/upmendex/mendex.h:9:28: fatal error: unicode/utypes.h: No such file or directory
 #include <unicode/utypes.h>

This file is in libs/icu/icu-56.1/source/common/unicode/utypes.h
or Work/libs/icu/include/unicode/utypes.h and neither of them
have a corresponding -I option.