dependencies: additional check for PERL_MM_OPT

When the user has Perl local::lib installed, its environment contains
a PERL_MM_OPT variables which defines
INSTALL_BASE="/some/location". Unfortunately, having this makes the
build of libxml-parser-perl (and probably other Perl related packages)
fail, because it complains that both PREFIX (passed in our .mk file)
and INSTALL_BASE (passed from the PERL_MM_OPT environment variable)
are defined. Since in Buildroot we want our PREFIX to be considered,
we error out when the user has PERL_MM_OPT defined.

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