When MAKEOBJDIRPREFIX points to a case-insensitive file system, the

When MAKEOBJDIRPREFIX points to a case-insensitive file system, the
build can break when different source files create the same target
files (case-insensitivity speaking).  This is the case for object
files compiled with -fpic and shared libraries. The former uses
an extension of ".So", and the latter an extension ".so".  Rename
shared object files from *.So to *.pico to match what NetBSD does.

See also r305855

MFC after:	1 month
Sponsored by:	Bracket Computing
Differential Revision:	https://reviews.freebsd.org/D7906