error: command 'clang' failed with exit status 1

[Deleted user]

pip install j2cli[yaml]

gives

    building '_yaml' extension
    creating build/temp.macosx-10.14-x86_64-3.7/ext
    clang -Wno-unused-result -Wsign-compare -Wunreachable-code -fno-common -dynamic -DNDEBUG -g -fwrapv -O3 -Wall -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk/usr/include -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.14.sdk/System/Library/Frameworks/Tk.framework/Versions/8.5/Headers -I/usr/local/include -I/usr/local/opt/openssl/include -I/usr/local/opt/sqlite/include -I/usr/local/Cellar/python/3.7.3/Frameworks/Python.framework/Versions/3.7/include/python3.7m -c ext/_yaml.c -o build/temp.macosx-10.14-x86_64-3.7/ext/_yaml.o
    In file included from ext/_yaml.c:343:
    ext/_yaml.h:10:9: warning: 'PyString_CheckExact' macro redefined [-Wmacro-redefined]
    #define PyString_CheckExact PyBytes_CheckExact
            ^
    ext/_yaml.c:184:11: note: previous definition is here
      #define PyString_CheckExact          PyUnicode_CheckExact
              ^
    ext/_yaml.c:1346:17: warning: assigning to 'char *' from 'const char *' discards qualifiers [-Wincompatible-pointer-types-discards-qualifiers]
      __pyx_v_value = yaml_get_version_string();
                    ^ ~~~~~~~~~~~~~~~~~~~~~~~~~

Python 3.7.3, Mojave

[Mark]

Wow, that was stupid of me. Fixed it, thank you for your vigilance!
I'll publish a release shortly.