buildsys: always link gap against libgap

... even the copy we don't install system wide.

On macOS this allows us an annoying issue with the linker flags used by e.g.
gac for building kernel extensions: by always linking them against libgap we
don't need to use a flat namespace, nor rely on a bundle loader (which breaks
if you try to load the kernel extension in another binary using libgap).

Makefile.rules

@@ -308,10 +308,12 @@ ifneq (,$(findstring cygwin,$(host_os)))
 else
   ifneq (,$(findstring darwin,$(host_os)))
     GAC_CFLAGS = -fno-common
-    GAC_LDFLAGS = -bundle -undefined dynamic_lookup -Wl,-no_fixup_chains
+    GAC_LDFLAGS = -bundle -L$(abs_builddir) -lgap
+    GAC_LDFLAGS_FOR_INSTALL = -bundle -L$(libdir) -lgap
   else
     GAC_CFLAGS = -fPIC
-    GAC_LDFLAGS = -shared -fPIC
+    GAC_LDFLAGS = -shared -fPIC -L$(abs_builddir) -lgap
+    GAC_LDFLAGS_FOR_INSTALL = -shared -fPIC -L$(libdir) -lgap
   endif
 endif
 
@@ -509,8 +511,8 @@ libgap$(SHLIB_EXT): $(LIBGAP_FULL)
 	ln -sf $< $@
 
 # build rule for the main gap executable
-gap$(EXEEXT): build/obj/src/main.c.o $(OBJS) cnf/GAP-LDFLAGS cnf/GAP-LIBS cnf/GAP-OBJS
-	$(QUIET_LINK)$(LINK) $(GAP_LDFLAGS) $< $(OBJS) $(GAP_LIBS) -o $@
+gap$(EXEEXT): build/obj/src/main.c.o libgap$(SHLIB_EXT) cnf/GAP-LDFLAGS cnf/GAP-LIBS cnf/GAP-OBJS
+	$(QUIET_LINK)$(LINK) $(GAP_LDFLAGS) $< $(GAP_LIBS) -L${abs_builddir} -lgap -o $@
 
 # generate a modified copy of main.c for use by the `gap-install` binary
 build/main.c: src/main.c config.status
@@ -673,6 +675,7 @@ install-sysinfo: SYSINFO_CPPFLAGS = -I${includedir}/gap -I${includedir} $(GAP_DE
 install-sysinfo: SYSINFO_LDFLAGS = $(ABI_CFLAGS)
 install-sysinfo: SYSINFO_GAP = $(bindir)/gap
 install-sysinfo: SYSINFO_GAC = $(bindir)/gac
+install-sysinfo: GAC_LDFLAGS = $(GAC_LDFLAGS_FOR_INSTALL)
 install-sysinfo: GMP_PREFIX =
 install-sysinfo:
 	$(INSTALL) -d -m 0755 $(DESTDIR)$(libdir)/gap