How to link with C++ using libtool? - makefile

Question

SQlite seems to use an own varian of libtool. Now I have added an extension to SQLite depends on some C++ code. However, I'm not able to link the C and C++ code. The error I get is:
c++ -Wl,-undefined,error -o libsqlite3.la sqlite3.lo -lz extsql.lo \
-rpath "/usr/local/lib"
ld: warning: ignoring file sqlite3.lo, building for macOS-x86_64 but attempting to link with file built for unknown-unsupported file format ( 0x23 0x20 0x73 0x71 0x6C 0x69 0x74 0x65 0x33 0x2E 0x6C 0x6F 0x20 0x2D 0x20 0x61 )
ld: warning: ignoring file extsql.lo, building for macOS-x86_64 but attempting to link with file built for unknown-unsupported file format ( 0x23 0x20 0x65 0x64 0x67 0x65 0x73 0x71 0x6C 0x2E 0x6C 0x6F 0x20 0x2D 0x20 0x61 )
Undefined symbols for architecture x86_64:
"_main", referenced from:
implicit entry/start for main executable
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
Thesee are my changes to the Makefile.in:
LTCOMPILECXX = $(LIBTOOL) --mode=compile --tag=CXX $(TCC) $(LTCOMPILE_EXTRAS)
LTLINKCXX = $(LIBTOOL) --tag=CXX --mode=link $(TCXX) $(LTCOMPILE_EXTRAS) #LDFLAGS# $(LTLINK_EXTRAS)
libsqlite3.la: $(LIBOBJ)
$(CXX) -Wl,-undefined,error -o $# $(LIBOBJ) $(TLIBS) \
${ALLOWRELEASE} -rpath "$(libdir)"
libtclsqlite3.la: tclsqlite.lo libsqlite3.la
$(CXX) -Wl,-undefined,error -o $# tclsqlite.lo \
libsqlite3.la #TCL_STUB_LIB_SPEC# $(TLIBS) \
-rpath "$(TCLLIBDIR)" \
-avoid-version
extsql$(TEXE): shell.c libsqlite3.la
$(CXX) $(READLINE_FLAGS) $(SHELL_OPT) -o $# \
shell.c libsqlite3.la \
$(LIBREADLINE) $(TLIBS) -rpath "$(libdir)"
extsql.lo: $(TOP)/extsql/extsql extsql_ext.h
$(LTCOMPILECXX) -c $(TOP)/extsql/extsql.cpp
Note the use of CXX instead of the original LTLINK command for the original makefile:
libsqlite3.la: $(LIBOBJ)
$(LTLINK) -no-undefined -o $# $(LIBOBJ) $(TLIBS) \
${ALLOWRELEASE} -rpath "$(libdir)" -version-info "8:6:8"
libtclsqlite3.la: tclsqlite.lo libsqlite3.la
$(LTLINK) -no-undefined -o $# tclsqlite.lo \
libsqlite3.la #TCL_STUB_LIB_SPEC# $(TLIBS) \
-rpath "$(TCLLIBDIR)" \
-version-info "8:6:8" \
-avoid-version
sqlite3$(TEXE): shell.c sqlite3.c
$(LTLINK) $(READLINE_FLAGS) $(SHELL_OPT) -o $# \
shell.c sqlite3.c \
$(LIBREADLINE) $(TLIBS) -rpath "$(libdir)"
The original makefile can be found here:
https://github.com/sqlite/sqlite/blob/branch-3.39/Makefile.in