Revert "buildsys: use the C++ compiler for linking" (#3717)

* Revert "buildsys: use the C++ compiler for linking"

This reverts commit 57c57c2.

* AppVeyor: use correct CXXFLAGS

* AppVeyor: print gcc version to ease debugging

* buildsys: use -fno-exceptions -fno-rttit for our C++ code

We disable support for exceptions and RTTI as we don't use them and they cause
compiler/linker issues in some build configurations; but we are careful to not
put these into GAP_CXXFLAGS, as kernel extensions may want to use GAP_CXXFLAGS
but also may have a  need to interface with C++ code in libraries that use
exceptions.

.appveyor.yml

@@ -42,6 +42,7 @@ build_script:
         throw "There are newer queued builds for this pull request, failing early." }
   - SET "PATH=%CYG_ROOT%\bin;%PATH%"
   - python -m pip install gcovr # for coverage reporting later on
+  - bash -lc "gcc --version"
   - bash -lc "cd $APPVEYOR_BUILD_FOLDER && ./etc/ci-prepare.sh"
 
 test_script:

.travis.yml

@@ -144,7 +144,6 @@ matrix:
 
     # test Julia integration
     - env: TEST_SUITES="testinstall" JULIA=yes CONFIGFLAGS="--disable-Werror"
-      dist: bionic # need GCC 6 to avoid linker error
 
 script:
   - set -e

Makefile.rules

@@ -342,9 +342,15 @@ $(srcdir)/src/pperm.c:
 $(srcdir)/src/trans.c:
 
 # Build rule for C++ source files
+#
+# We disable support for exceptions and RTTI as we don't use them and they
+# cause compiler/linker issues in some build configurations; but we are
+# careful to not put these into GAP_CXXFLAGS, as kernel extensions may want to
+# use GAP_CXXFLAGS but also may have a  need to interface with C++ code in
+# libraries that use exceptions.
 obj/%.lo: %.cc cnf/GAP-CXXFLAGS cnf/GAP-CPPFLAGS libtool
 	@$(MKDIR_P) $(@D)/$(DEPDIRNAME)
-	$(QUIET_CXX)$(LIBTOOL) --mode=compile --tag CXX $(CXX) $(DEPFLAGS) $(GAP_CXXFLAGS) $(WARN_CXXFLAGS) $(GAP_CPPFLAGS) -c $< -o $@
+	$(QUIET_CXX)$(LIBTOOL) --mode=compile --tag CXX $(CXX) $(DEPFLAGS) $(GAP_CXXFLAGS) -fno-exceptions -fno-rtti $(WARN_CXXFLAGS) $(GAP_CPPFLAGS) -c $< -o $@
 	@echo "$<:" >> $(DEPFILE)
 
 # Build rule for C source files
@@ -358,7 +364,7 @@ obj/%.lo: %.c cnf/GAP-CFLAGS cnf/GAP-CPPFLAGS libtool
 # Linker rules for gap executable
 ########################################################################
 
-LINK=$(LIBTOOL) --mode=link $(CXX) -export-dynamic
+LINK=$(LIBTOOL) --mode=link $(CC) -export-dynamic
 
 libgap.la: $(OBJS) cnf/GAP-LDFLAGS cnf/GAP-LIBS cnf/GAP-OBJS
 	$(QUIET_LINK)$(LINK) -no-undefined -rpath $(libdir) $(GAP_LDFLAGS) $(OBJS) $(GAP_LIBS) -o $@
@@ -391,8 +397,8 @@ GAP_LDFLAGS += -Wl,--allow-multiple-definition
 all: bin/$(GAPARCH)/gap.dll
 bin/$(GAPARCH)/gap.dll: symlinks libgap.la
 	cp .libs/cyggap-0.dll $@  # FIXME: HACK to support kernel extensions
-gap$(EXEEXT): obj/src/gapw95.lo libgap.la cnf/GAP-LDFLAGS cnf/GAP-LIBS cnf/GAP-OBJS
-	$(QUIET_LINK)$(LINK) $(GAP_LDFLAGS) obj/src/gapw95.lo $(GAP_LIBS) libgap.la -o $@
+gap$(EXEEXT): libgap.la cnf/GAP-LDFLAGS cnf/GAP-LIBS cnf/GAP-OBJS
+	$(QUIET_LINK)$(LINK) $(GAP_LDFLAGS) $(srcdir)/src/gapw95.c $(GAP_LIBS) libgap.la -o $@
 	@( if which peflags > /dev/null ; then peflags --cygwin-heap=2048 gap$(EXEEXT) ; fi )
 
 else
@@ -694,9 +700,6 @@ endif # BUILD_BOEHM_GC
 # ensure subprojects are built and "installed" before compiling and linking GAP
 gap$(EXEEXT): $(EXTERN_FILES)
 $(OBJS): $(EXTERN_FILES)
-ifeq ($(SYS_IS_CYGWIN32),yes)
-obj/src/gapw95.lo: $(EXTERN_FILES)
-endif
 ifeq ($(HPCGAP),yes)
 $(SOURCES): $(EXTERN_FILES)
 endif