support/script/scancpan: add support for BR2_PREFER_STATIC_LIB

Perl extensions are loaded at runtime with dlopen(), so it does not
make sense to even build extensions that are written in C when
BR2_PREFER_STATIC_LIB is enabled. A Perl module written in C or with a
dependency on a module written in C is not available when doing a
static build.

Therefore, this commit adapts the scancpan script to automatically
generate a dependency on !BR2_PREFER_STATIC_LIB when the Perl module
would not work in a static-only configuration.

Signed-off-by: Francois Perrad <francois.perrad@gadz.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>