fltk: fix build issue caused by rpath

The fltk build system has some logic that causes it to pass
-Wl,-rpath,/usr/lib when --libdir is not /usr/lib. However, in our
case, libdir is ${exec_prefix}/lib, and is not expanded to /usr/lib
before the rpath related test is done. Rather than fixing the fltk
build system, this commit works around the problem by explicitly
passing --libdir=/usr/lib.

Fixes:

  http://autobuild.buildroot.org/results/8d1/8d1b202a182e3fb5dee21f20afc9f749c2defa1a/



and many other similar build failures that have been occuring since 1+
year.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>