add qt5-host-tools example plugin

starting point for #950, builds `qmake.exe`, `rcc.exe`, etc

plugins/examples/host-toolchain/qt5-host-tools.mk

@@ -0,0 +1,52 @@
+# This file is part of MXE.
+# See index.html for further information.
+
+PKG             := $(basename $(notdir $(lastword $(MAKEFILE_LIST))))
+$(PKG)_FILE      = $(qtbase_FILE)
+$(PKG)_PATCHES   = $(realpath $(sort $(wildcard $(addsuffix /qtbase-[0-9]*.patch, $(TOP_DIR)/src))))
+$(PKG)_SUBDIR    = $(qtbase_SUBDIR)
+$(PKG)_DEPS     := gcc qtbase
+
+# main configure options: -platform -host-option -external-hostbindir
+# further testing needed: -prefix -extprefix -hostprefix -sysroot -no-gcc-sysroot
+#                         and keeping options synced with qtbase
+
+define $(PKG)_BUILD
+    $(SED) -i 's,BUILD_ON_MAC=yes,BUILD_ON_MAC=no,g' '$(1)/configure'
+    mkdir '$(1).build'
+    cd    '$(1).build' && '$(1)/configure' \
+            -prefix '$(PREFIX)/$(TARGET)/$(PKG)' \
+            -static \
+            -release \
+            -c++std c++11 \
+            -platform win32-g++ \
+            -host-option CROSS_COMPILE=${TARGET}- \
+            -external-hostbindir '$(PREFIX)/$(TARGET)/qt5/bin' \
+            -device-option PKG_CONFIG='${TARGET}-pkg-config' \
+            -device-option CROSS_COMPILE=${TARGET}- \
+            -force-pkg-config \
+            -no-sql-{db2,ibase,mysql,oci,odbc,psql,sqlite,sqlite2,tds} \
+            -no-use-gold-linker \
+            -nomake examples \
+            -nomake tests \
+            -opensource \
+            -confirm-license \
+            -continue \
+            -verbose
+
+    rm -rf '$(PREFIX)/$(TARGET)/$(PKG)'
+    # install qmake.exe (created by configure)
+    # and generate remaining build configuration
+    $(MAKE) -C '$(1).build' -j $(JOBS) \
+        sub-qmake-qmake-aux-pro-install_subtargets \
+        sub-src-qmake_all
+
+    # build and install other tools
+    $(MAKE) -C '$(1).build/src' -j $(JOBS) \
+        sub-moc-install_subtargets \
+        sub-qdbuscpp2xml-install_subtargets \
+        sub-qdbusxml2cpp-install_subtargets \
+        sub-qlalr-install_subtargets \
+        sub-rcc-install_subtargets \
+        sub-uic-install_subtargets
+endef