qtbase: prevent extra leading / on system include and lib directories

PKG_CONFIG_SYSROOT should not be set because MXE uses fully
qualified paths. However, to  satisfy the Qt build system's
cross-compiling "sanity check", qtbase.mk defines
    PKG_CONFIG_SYSROOT_DIR=/
This causes pkg-config to prefix the system include and library
paths with an extra /. Pkg-config does not strip these prefixed
paths from --cflags and --libs output. The workaround here is
to remove the PKG_CONFIG_SYSROOT_DIR definition in the wrapper
script for pkg-config.

src/pkgconf.mk

@@ -22,6 +22,7 @@ define $(PKG)_BUILD
     # create pkg-config script
     (echo '#!/bin/sh'; \
      echo 'PKG_CONFIG_PATH="$(PREFIX)/$(TARGET)/qt5/lib/pkgconfig":"$$PKG_CONFIG_PATH_$(subst .,_,$(subst -,_,$(TARGET)))" \
+           PKG_CONFIG_SYSROOT_DIR= \
            PKG_CONFIG_LIBDIR="$(PREFIX)/$(TARGET)/lib/pkgconfig" \
            PKG_CONFIG_SYSTEM_INCLUDE_PATH="$(PREFIX)/$(TARGET)/include" \
            exec "$(PREFIX)/$(BUILD)/bin/pkgconf" $(if $(BUILD_STATIC),--static) "$$@"') \