configure.ac: fix cross-compilation without dejavu

configure.ac: fix cross-compilation without dejavu

Cross-compilation is broken since commit 1d66106. Indeed, when
cross-compiling, dejavu font won't be installed in /usr/share/fonts. The
path (e.g. $(TARGET_DIR)/usr/share/fonts/dejavu/DejaVuSans.ttf) could be
provided through --with-test-font-path which will fix the build failure
but will result in broken test programs.

So when cross-compiling don't try to check if the test-font-path is
valid or not and just use the one provided by the user or the default
one (e.g. /usr/share/fonts/dejavu/DejaVuSans.ttf)

Fix #262

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>