fate: get samples location from env var if not explicitly set

Use the FATE_SAMPLES environment variable if samples location
is not set with the --samples configure option or on the make
command line.
Signed-off-by: Mans Rullgard <mans@mansr.com>
(cherry picked from commit aa3805a4)

configure

@@ -3258,7 +3258,7 @@ SLIB_CREATE_DEF_CMD=${SLIB_CREATE_DEF_CMD}
 SLIB_EXTRA_CMD=${SLIB_EXTRA_CMD}
 SLIB_INSTALL_EXTRA_CMD=${SLIB_INSTALL_EXTRA_CMD}
 SLIB_UNINSTALL_EXTRA_CMD=${SLIB_UNINSTALL_EXTRA_CMD}
-SAMPLES=$samples
+SAMPLES=${samples:-\$(FATE_SAMPLES)}
 EOF
 
 get_version(){