Makefile: make $(BUILD_DIR)/.root rule idempotent

The $(BUILD_DIR)/.root rule is executed as part of the 'dirs'
target. The 'dirs' target is re-executed at every execution of 'make
external-deps', and make external-deps explicitly tells make to ignore
targets that have already been made (through the -B option). This
means that the $(BUILD_DIR)/.root rule has to be idempotant, which was
not the case this the introduction of the lib32/lib64 symbolic link.

Running 'make external-deps' three times in a row was sufficient to
trigger an error due to symbolic links being incorrectly created. This
patch fixes that.

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