depends: qt: Fix {C{,XX},LD}FLAGS pickup

Summary:
Backport of core [[bitcoin/bitcoin#16837 | PR16837]].

Depends on D5650.

Test Plan: Run the Gitian builds.

Reviewers: #bitcoin_abc, deadalnix

Reviewed By: #bitcoin_abc, deadalnix

Differential Revision: https://reviews.bitcoinabc.org/D5656

depends/packages/qt.mk

@@ -172,9 +172,9 @@ define $(package)_preprocess_cmds
   patch -p1 -i $($(package)_patch_dir)/no-xlib.patch &&\
   echo "QMAKE_LINK_OBJECT_MAX = 10" >> qtbase/mkspecs/win32-g++/qmake.conf &&\
   echo "QMAKE_LINK_OBJECT_SCRIPT = object_script" >> qtbase/mkspecs/win32-g++/qmake.conf &&\
-  sed -i.old "s|QMAKE_CFLAGS            = |!host_build: QMAKE_CFLAGS            = $($(package)_cflags) $($(package)_cppflags) |" qtbase/mkspecs/win32-g++/qmake.conf && \
-  sed -i.old "s|QMAKE_LFLAGS            = |!host_build: QMAKE_LFLAGS            = $($(package)_ldflags) |" qtbase/mkspecs/win32-g++/qmake.conf && \
-  sed -i.old "s|QMAKE_CXXFLAGS          = |!host_build: QMAKE_CXXFLAGS            = $($(package)_cxxflags) $($(package)_cppflags) |" qtbase/mkspecs/win32-g++/qmake.conf
+  sed -i.old "s|QMAKE_CFLAGS            += |!host_build: QMAKE_CFLAGS            = $($(package)_cflags) $($(package)_cppflags) |" qtbase/mkspecs/win32-g++/qmake.conf && \
+  sed -i.old "s|QMAKE_CXXFLAGS          += |!host_build: QMAKE_CXXFLAGS            = $($(package)_cxxflags) $($(package)_cppflags) |" qtbase/mkspecs/win32-g++/qmake.conf && \
+  sed -i.old "0,/^QMAKE_LFLAGS_/s|^QMAKE_LFLAGS_|!host_build: QMAKE_LFLAGS            = $($(package)_ldflags)\n&|" qtbase/mkspecs/win32-g++/qmake.conf
 endef
 
 define $(package)_config_cmds