erlang: fix incorrect use of AC_EGREP_CPP

Using 'AC_EGREP_CPP(yes' without restraining the pattern always return
true if it runs from a path containing the string 'yes'.

Signed-off-by: Johan Oudinet <johan.oudinet@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>