-D_WIN64 breaks build

[Ivan Gyurdiev]

utf8.h includes umachine.h
umachine.h detects _WIN64 definition and includes pwin32.h
pwin32.h defines U_EXPORT to __declspec(dllexport)
...later U_INTERNAL includes U_EXPORT in its definition

so eventually it errors out:

gcc -c -I. -I. -I../../include -I../../include -I/usr/include/freetype2 
-D__WINESRC__ -D_GDI32_ -D_REENTRANT -fPIC -Wall -pipe 
-fno-strict-aliasing -gstabs+ -Wdeclaration-after-statement 
-Wwrite-strings -Wpointer-arith -g -O2 -D_WIN64 -o bidi.o bidi.c
In file included from /usr/include/unicode/utf.h:222,
from /usr/include/unicode/utypes.h:38,
from /usr/include/unicode/ubidi.h:21,
from bidi.c:27:
/usr/include/unicode/utf8.h: In function ‘__declspec’:
/usr/include/unicode/utf8.h:75: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or 
‘__attribute__’ before ‘UChar32’
/usr/include/unicode/utf8.h:82: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or 
‘__attribute__’ before ‘int32_t’
...