Remove compile-time dependencies on BasiliskII

[ianfixes]

ag BasiliskII in this repo yields some concerning entries:

Makefile
16:B2_TOPDIR := ../BasiliskII
37:	cd $(TMPDIR); cvs export -D "$(ISODATE)" BasiliskII SheepShaver

src/MacOSX/XcodeBuildHowTo.txt
15:1. A complete 'macemu' git checkout of SheepShaver and BasiliskII.

src/MacOSX/SheepShaver_Xcode8.xcodeproj/project.pbxproj
154:		082AC26114AA59F000071F5E /* lowmem.c */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.c; name = lowmem.c; path = ../../../BasiliskII/src/Unix/Darwin/lowmem.c; sourceTree = SOURCE_ROOT; };

src/MacOSX/SheepShaver.xcodeproj/project.pbxproj
220:		082AC26114AA59F000071F5E /* lowmem.c */ = {isa = PBXFileReference; fileEncoding = 4; lastKnownFileType = sourcecode.c.c; name = lowmem.c; path = ../../../BasiliskII/src/Unix/Darwin/lowmem.c; sourceTree = SOURCE_ROOT; };

Duplicate files as necessary to make these projects stand alone. Open other issues as necessary if we need to create a shared component (as a separate repo).