linux.370 hints: validate sysconf GDBPATH

With sys/unix/hints/linux.370, if GDBPATH doesn't exist, comment out
the GDBPATH line in sysconf during 'make install' or 'make update'.

[ macOS apparently uses sys/unix/hints/macosx.sh to do its sysconf manipulation,
so there is no corresponding change for sys/unix/hints/macOS.370 ]

Closes #1477

sys/unix/Makefile.top

@@ -440,6 +440,8 @@ update: $(PRECHECK) $(GAME) recover $(VARDAT) spec_levs sys/unix/sysconf
 	touch $(VARDIR)/perm $(VARDIR)/record
 # sysconf, but only if it does not exist
 	true; $(SYSCONFENSURE)
+# other steps from hints file
+	true; $(POSTUPDATE)
 # and a reminder
 	@echo You may also want to install the man pages via the doc Makefile.
 

sys/unix/hints/linux.370

@@ -390,13 +390,15 @@ endif #?WANT_SOURCE_INSTALL
 INSTDIR=$(HACKDIR)
 VARDIR = $(HACKDIR)
 
-ifneq "$(CCISCLANG)" ""
-# gdb may not be installed if clang is chosen compiler so the game
-# won't start in that case due to a sysconf error. Comment out
-# relevant lines in sysconf.
-POSTINSTALL+= sed -i -e 's;^GDBPATH=/usr/bin/gdb;\#GDBPATH=/usr/bin/gdb;' \
+ifdef MAKEFILE_TOP
+TESTGDBPATH=/usr/bin/gdb
+POSTINSTALL+= test -f $(TESTGDBPATH) || \
+		sed -i -e 's;^GDBPATH=/usr/bin/gdb;\#GDBPATH=/usr/bin/gdb;' \
 		-e 's;PANICTRACE_GDB=1;PANICTRACE_GDB=0;' $(INSTDIR)/sysconf;
-endif
+POSTUPDATE+= test -f $(TESTGDBPATH) || \
+		sed -i -e 's;^GDBPATH=/usr/bin/gdb;\#GDBPATH=/usr/bin/gdb;' \
+		-e 's;PANICTRACE_GDB=1;PANICTRACE_GDB=0;' $(INSTDIR)/sysconf;
+endif  #MAKEFILE_TOP
 
 ifeq '$(USE_NONOSTATICFN)' '1'
 CFLAGS += -DNONOSTATICFN