Fixes searching for native qt5 progs on cygwin and msys2

The reason for the issue is that on windows if there is a space in the path
or if the program is prepends with .exe and we specify the search PATH
in AC_PATH_PROGS we must enclose them in quotes or the program will not be
found.

For future reference, if anyone uses AC_PATH_PROGS please enclose defined
search paths (if you have any) in quotes
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>