[tlbuild] Tweak for harfbuzz

[Ken Brown]

I just tried a build of the current svn sources on Cygwin, and I needed 
the following tweak to make harfbuzz build:

Index: Build/source/libs/harfbuzz/configure.ac
===================================================================
--- Build/source/libs/harfbuzz/configure.ac     (revision 46123)
+++ Build/source/libs/harfbuzz/configure.ac     (working copy)
@@ -44,7 +44,7 @@
         LDFLAGS="$LDFLAGS -Bsymbolic-functions"

         # Choose C++ version
-       CXXFLAGS="$CXXFLAGS -std=c++11"
+       CXXFLAGS="$CXXFLAGS -std=c++11 -D_GNU_SOURCE"

         # Make sure we don't link to libstdc++
         CXXFLAGS="$CXXFLAGS -fno-rtti -fno-exceptions"


This is needed because -std=c++11 disables some locale-related POSIX and 
GNU extensions that harfbuzz uses.  Builders on glibc platforms don't 
notice this issue because gcc and g++ define _GNU_SOURCE unconditionally 
there.

Further information can be found starting here for anyone who's interested:

   https://sourceware.org/ml/newlib/2017/msg01303.html

Ken