26.1: build on macOS failed

[Zhang Haijun]

OS: macOS 10.13.6
Xcode: 10.1

Configuration:
CFLAGS="-O2" ./configure --without-makeinfo --with-ns --with-modules

Error message of make:

...
/Applications/Xcode.app/Contents/Developer/usr/bin/make -C lib-src all
  CC       regex.o
  CCLD     etags
ld: warning: ignoring file ../lib/libgnu.a, file was built for archive which is 
not th
e architecture being linked (x86_64): ../lib/libgnu.a
Undefined symbols for architecture x86_64:
  "_c_isalnum", referenced from:
      _Asm_labels in etags-59ad2c.o
      _Cobol_paragraphs in etags-59ad2c.o
      _Erlang_functions in etags-59ad2c.o
      _Perl_functions in etags-59ad2c.o
      _Prolog_functions in etags-59ad2c.o
      _Ruby_functions in etags-59ad2c.o
      _Ada_getit in etags-59ad2c.o
      ...
  "_c_strcasecmp", referenced from:
      _get_language_from_filename in etags-59ad2c.o
  "_c_strncasecmp", referenced from:
      _Forth_words in etags-59ad2c.o
      _Fortran_functions in etags-59ad2c.o
      _HTML_labels in etags-59ad2c.o
  "_rpl_getopt_long", referenced from:
      _main in etags-59ad2c.o
  "_rpl_optarg", referenced from:
      _main in etags-59ad2c.o
  "_rpl_optind", referenced from:
      _main in etags-59ad2c.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1 (use -v to see invocation)
make[1]: *** [etags] Error 1
make: *** [lib-src] Error 2