target QMAKE_DEFAULT_LIBDIRS is used for host builds

I'm cross-compiling Qt from Fedora to Windows using Fedora's mingw toolchain.

I've encountered a number of build failure when compiling host tools (i.e. lupdate, lrelease, etc) due to
LIBS = $(SUBLIBS) -lQt5Bootstrap -lpthread
missing the host library path
-L/usr/i686-w64-mingw32/lib

Investigating I've discovered the following:

• Module libdirs are filtered by QMAKE_DEFAULT_LIBDIRS:
  qt.prf: MODULE_LIBS_ADD -= $$QMAKE_DEFAULT_LIBDIRS

• QMAKE_DEFAULT_LIBDIRS is assembled in toolchain.prf, on my Fedora system based on the output of
  LC_ALL=C i686-w64-mingw32-g++ -O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 --param=ssp-buffer-size=4 -fno-keep-inline-dllexport -xc++ -E -v - 2>&1 </dev/null >/dev/null

• The output of the above command contains
  LIBRARY_PATH=/usr/lib/gcc/i686-w64-mingw32/6.3.0/:/usr/lib/gcc/i686-w64-mingw32/6.3.0/../../../../i686-w64-mingw32/lib/../lib/:/usr/i686-w64-mingw32/sys-root/mingw/lib/../lib/:/usr/lib/gcc/i686-w64-mingw32/6.3.0/../../../../i686-w64-mingw32/lib/:/usr/i686-w64-mingw32/sys-root/mingw/lib/

• Some of the entries in LIBRARY_PATH resolve to /usr/i686-w64-mingw32/lib which is the host library directory (as opposed to /usr/i686-w64-mingw32/sys-root/mingw/lib/ which is the target directory).

My conclusion is that QMAKE_DEFAULT_LIBDIRS should not contain the host library directory. Indeed a hacky patch along the lines of

--- a/mkspecs/features/toolchain.prf
+++ b/mkspecs/features/toolchain.prf
@@ -30,6 +30,12 @@ isEmpty(QMAKE_DEFAULT_INCDIRS):!host_build {
             }
         }
         QMAKE_DEFAULT_LIBDIRS = $$unique(QMAKE_DEFAULT_LIBDIRS)
+        #HACK Don't include the native libdir to the default libdirs when crosscompiling
+        #Fixes -lQt5Bootstrap not found due to missing -L (module libdirs are filtered by QMAKE_DEFAULT_LIBDIRS)
+        cross_compile: {
+            QMAKE_DEFAULT_LIBDIRS -= $$[QT_HOST_LIBS]
+        }
+
     }
 
     unix {

fixes the build failures.

I'm not sure if my analysis is correct and if this is possibly a bug in the toolchain, i.e. that the host directory should not even be listed in LIBRARY_PATH. Reporting here to get the opinion of someone more familiar with the qmake internals.