Gitian win32 build broken #3522

A gitian win32 build against master fails. It appears to have to do with Qt, so I think it is a regression from #3346.

From build.log:

checking if QT should be enabled... no (Qt4)
configure: WARNING: zlib not found; bitcoin-qt frontend will not be built

....

make -C ./src/qt bitcoin-qt.exe
make[1]: Entering directory `/home/ubuntu/build/bitcoin/distsrc/src/qt'
  CXX    bitcoin_qt-bitcoin.o
In file included from paymentserver.h:34:0,
                 from bitcoin.cpp:18:
paymentrequestplus.h:8:31: fatal error: paymentrequest.pb.h: No such file or directory
compilation terminated.
make[1]: make[1]: Leaving directory `/home/ubuntu/build/bitcoin/distsrc/src/qt'
*** [bitcoin_qt-bitcoin.o] Error 1

@theuni Seems it is missing zlib for some reason?

Edit: this is the failing check https://github.com/bitcoin/bitcoin/blob/master/src/m4/bitcoin_qt.m4#L318 From inspection of config.log it looks like it can't link against -lz.

Edit2: removing the explicit check for libz solves the build issue . Why is there an explicit check for lz in there in the first place?

@laanwj Ah, yep. I have a libz in my win32 environment, so I didn't notice.

libz is optional for qt4, mandatory for qt5. I'll PR a fix.

@theuni I actually need that line now that I use Qt5 and with system libz/libpng, so consider this fixed.