imagemagick: fix magick++-config fixup error

Magic++ bindings are built only with C++ and WCHAR toolchains.
Add a WCHAR toolchain check for the magick++ config fixup.
Looking into the future the fixup shouldn't bail on a missing file so we
can avoid awkward kludges for packages that have many options and config
files.
Fixes:
http://autobuild.buildroot.net/results/33a/33ac4b17866a64379b7bab3c0549f6e075c98dde/



Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>