dependencies: ensure that DESTDIR isn't set when running Buildroot

Having DESTDIR set in the environment before running Buildroot creates
some funky problems in the build process. Prevent users from running
into this kind of troubles.

Cc: Michael Opdenacker <michael.opdenacker@free-electrons.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>

support/dependencies/dependencies.sh

@@ -48,6 +48,12 @@ if test -n "$PERL_MM_OPT" ; then
     exit 1
 fi
 
+if test -n "$DESTDIR" ; then
+    /bin/echo -e "\nYou have the DESTDIR environment variable set. Please"
+    /bin/echo -e "unset it so that Buildroot can work properly."
+    exit 1
+fi
+
 # Verify that which is installed
 if ! which which > /dev/null ; then
 	/bin/echo -e "\nYou must install 'which' on your build machine\n";