Two minor Makefile.SH cleanups

Use "$osname" instead of (bare) $osname in all case statements.
This was the only one that was different.

Delete PATH_SEP, which was never used.
This was added in Nov 1995 as part of commit 4633a7c:
    5.002 beta 1

with various other macros, and the comment "These variables will be used in
a future version to make the make file more portable to non-unix systems."

The other macros have been used, but not PATH_SEP.

Curiously it also gets a different value if one (re)generates Makefile by
running ./Makefile.SH, compared with a Makefile generated by Configure,
because the variable that it is set from - $p_ - is not in config.sh

Author: nwc10

Makefile.SH

@@ -335,7 +335,6 @@ HOST_EXE_EXT = $_exe
 EXE_EXT = $_exe
 LIB_EXT = $_a
 OBJ_EXT = $_o
-PATH_SEP = $p_
 
 # Macros to invoke a copy of miniperl during the build.  Targets which
 # are built using these macros should depend on \$(MINIPERL_EXE)
@@ -1024,7 +1023,7 @@ $(PERL_EXE): $& $(perlmain_dep) $(LIBPERL) $(static_ext) ext.libs $(PERLEXPORT)
 	-@rm -f miniperl.xok
 !NO!SUBS!
 
-        case $osname in
+        case "$osname" in
 	# In AmigaOS the Perl executable needs to be linked with -ldl,
 	# but none of the other executables should be.
         amigaos) $spitshell >>$Makefile <<'!NO!SUBS!'