chasen-base @2.3.3: can't build on macOS 10.14 Mojave

[chrstphrchvz (Christopher Chavez)]

First noticed this in an the Azure builds: ​https://paste.z0k.xyz/d52c7a4964e9 (the build for 10.13 High Sierra succeeded though: ​https://paste.z0k.xyz/701be7feaf54). I was able to reproduce this on my machine after disabling ccache (#58479); will attach main.log.

/usr/bin/clang++ -dynamiclib -flat_namespace -undefined suppress -o .libs/libchasen.0.0.1.dylib  block.lo chalib.lo connect.lo dartsdic.lo getid.lo getopt.lo grammar.lo init.lo iotool.lo jfgets.lo katuyou.lo lisp.lo literal.lo mmap.lo parse.lo print.lo tokenizer.lo  -L/opt/local/lib -liconv -lc -headerpad_max_install_names -syslibroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk -install_name  /opt/local/lib/libchasen.0.dylib -compatibility_version 1 -current_version 1.1
clang: error: unknown argument: '-syslibroot'