package/qt5/qt5base: use ccache for building host code

qt5 currently doesn't use HOSTCC/HOSTCXX, so it doesn't use ccache
when building all its host code (especially qmake). This means that
even with ccache enabled and a hot cache, it still takes a long time
to build qt5base.

Before this patch, building qt5base takes:

 - 446 seconds with a cold ccache
 - 185 seconds with a hot ccache

This is because the ccache is not used for host code.

After this patch, building qt5base takes:

 - 450 seconds with a cold ccache
 - 15 seconds with a hot ccache

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>

package/qt5/qt5base/qt5base.mk

@@ -271,6 +271,13 @@ define QT5BASE_CONFIGURE_ARCH_CONFIG
 endef
 endif
 
+# This allows to use ccache when available
+define QT5BASE_CONFIGURE_HOSTCC
+	$(SED) 's,^QMAKE_COMPILER\s*=.*,QMAKE_COMPILER=$(HOSTCC),' $(@D)/mkspecs/common/g++-base.conf
+	$(SED) 's,^QMAKE_CC\s*=.*,QMAKE_CC=$(HOSTCC),' $(@D)/mkspecs/common/g++-base.conf
+	$(SED) 's,^QMAKE_CXX\s*=.*,QMAKE_CXX=$(HOSTCXX),' $(@D)/mkspecs/common/g++-base.conf
+endef
+
 define QT5BASE_CONFIGURE_CMDS
 	mkdir -p $(@D)/mkspecs/devices/linux-buildroot-g++/
 	sed 's/@EGLFS_DEVICE@/$(QT5BASE_EGLFS_DEVICE)/g' \
@@ -281,6 +288,7 @@ define QT5BASE_CONFIGURE_CMDS
 	$(QT5BASE_CONFIGURE_CONFIG_FILE)
 	touch $(QT5BASE_ARCH_CONFIG_FILE)
 	$(QT5BASE_CONFIGURE_ARCH_CONFIG)
+	$(QT5BASE_CONFIGURE_HOSTCC)
 	(cd $(@D); \
 		$(TARGET_MAKE_ENV) \
 		PKG_CONFIG="$(PKG_CONFIG_HOST_BINARY)" \