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 +308,13 @@ ifneq (,$(findstring cygwin,$(host_os)))
 else
   ifneq (,$(findstring darwin,$(host_os)))
     GAC_CFLAGS = -fno-common
-    GAC_LDFLAGS = -bundle -flat_namespace -bundle_loader $(SYSINFO_GAP)
+    GAC_LDFLAGS = -bundle -L$(abs_builddir) -lgap
+    GAC_LDFLAGS_FOR_INSTALL = -bundle -L$(libdir) -lgap
+
   else
     GAC_CFLAGS = -fPIC
     GAC_LDFLAGS = -shared -fPIC
+    GAC_LDFLAGS_FOR_INSTALL = -shared -fPIC
   endif
 endif
 
@@ -492,13 +495,13 @@ GAP_LDFLAGS += -Wl,--allow-multiple-definition
 # loads that DLL and calls the renamed main function.
 all: bin/$(GAPARCH)/gap.dll
 
-bin/$(GAPARCH)/gap.dll: libgap.dll
+bin/$(GAPARCH)/gap.dll: libgap$(SHLIB_EXT)
 	@$(MKDIR_P) bin/$(GAPARCH)
 	cp $< $@
 
 # build rule for the main gap executable
-gap$(EXEEXT): libgap.dll cnf/GAP-LDFLAGS cnf/GAP-LIBS cnf/GAP-OBJS build/obj/src/main.c.o
-	$(QUIET_LINK)$(LINK) $(GAP_LDFLAGS) -Wl,--export-all-symbols build/obj/src/main.c.o $(GAP_LIBS) -L${abs_builddir} -lgap -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) -Wl,--export-all-symbols $< $(GAP_LIBS) -L${abs_builddir} -lgap -o $@
 	@( if which peflags > /dev/null ; then peflags --cygwin-heap=2048 gap$(EXEEXT) ; fi )
 
 else
@@ -509,17 +512,17 @@ libgap$(SHLIB_EXT): $(LIBGAP_FULL)
 	ln -sf $< $@
 
 # build rule for the main gap executable
-gap$(EXEEXT): $(OBJS) cnf/GAP-LDFLAGS cnf/GAP-LIBS cnf/GAP-OBJS build/obj/src/main.c.o
-	$(QUIET_LINK)$(LINK) $(GAP_LDFLAGS) build/obj/src/main.c.o $(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
 	@echo "#define SYS_DEFAULT_PATHS \"$(libdir)/gap;$(datarootdir)/gap\"" > $@
 	@cat $< >> $@
 
 # build rule for the gap executable used by the `install-bin` target
-build/gap-install: libgap$(SHLIB_EXT) cnf/GAP-LDFLAGS cnf/GAP-LIBS cnf/GAP-OBJS build/obj/build/main.c.o
-	$(QUIET_LINK)$(LINK) $(GAP_LDFLAGS) build/obj/build/main.c.o $(GAP_LIBS) -L${abs_builddir} -lgap -o $@
+build/gap-install: build/obj/build/main.c.o libgap$(SHLIB_EXT) cnf/GAP-LDFLAGS cnf/GAP-LIBS cnf/GAP-OBJS
+	$(QUIET_LINK)$(LINK) $(GAP_LDFLAGS) $< $(GAP_LIBS) -L${abs_builddir} -lgap -o $@
 	$(INSTALL_NAME_TOOL) -change $(LIBGAP_FULL) $(libdir)/$(LIBGAP_FULL) $@
 
 endif
@@ -673,6 +676,7 @@ 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