build: patch qt libpng to fix powerpc build

This is an alternative to bitcoin#19751 that fixes the build without requiring
splitting out libpng. This patch can be dropped once we are building qt
5.12.0 or later.

depends/packages/qt.mk

@@ -11,7 +11,7 @@ $(package)_qt_libs=corelib network widgets gui plugins testlib
 $(package)_patches=fix_qt_pkgconfig.patch mac-qmake.conf fix_configure_mac.patch fix_no_printer.patch
 $(package)_patches+= fix_rcc_determinism.patch fix_riscv64_arch.patch xkb-default.patch no-xlib.patch
 $(package)_patches+= fix_android_qmake_conf.patch fix_android_jni_static.patch dont_hardcode_pwd.patch
-$(package)_patches+= freetype_back_compat.patch drop_lrelease_dependency.patch
+$(package)_patches+= freetype_back_compat.patch drop_lrelease_dependency.patch fix_powerpc_libpng.patch
 
 # Update OSX_QT_TRANSLATIONS when this is updated
 $(package)_qttranslations_file_name=qttranslations-$($(package)_suffix)
@@ -194,6 +194,7 @@ endef
 
 define $(package)_preprocess_cmds
   patch -p1 -i $($(package)_patch_dir)/freetype_back_compat.patch && \
+  patch -p1 -i $($(package)_patch_dir)/fix_powerpc_libpng.patch && \
   sed -i.old "s|updateqm.commands = \$$$$\$$$$LRELEASE|updateqm.commands = $($(package)_extract_dir)/qttools/bin/lrelease|" qttranslations/translations/translations.pro && \
   patch -p1 -i $($(package)_patch_dir)/drop_lrelease_dependency.patch && \
   patch -p1 -i $($(package)_patch_dir)/dont_hardcode_pwd.patch &&\

depends/patches/qt/fix_powerpc_libpng.patch

@@ -0,0 +1,23 @@
+commit 6f9feb773a43c5abfa3455da2e324180e789285b
+Author: fanquake <fanquake@gmail.com>
+Date:   Tue Sep 15 21:44:31 2020 +0800
+
+    Fix PowerPC build of libpng
+
+    See https://bugreports.qt.io/browse/QTBUG-66388.
+
+    Can be dropped when we are building qt 5.12.0 or later.
+
+diff --git a/qtbase/src/3rdparty/libpng/libpng.pro b/qtbase/src/3rdparty/libpng/libpng.pro
+index 577b61d8..a2f56669 100644
+--- a/qtbase/src/3rdparty/libpng/libpng.pro
++++ b/qtbase/src/3rdparty/libpng/libpng.pro
+@@ -10,7 +10,7 @@ MODULE_INCLUDEPATH = $$PWD
+
+ load(qt_helper_lib)
+
+-DEFINES += PNG_ARM_NEON_OPT=0
++DEFINES += PNG_ARM_NEON_OPT=0 PNG_POWERPC_VSX_OPT=0
+ SOURCES += \
+     png.c \
+     pngerror.c \