On build fail due to `pkg-config` not being installed, error message is unhelpful

Created by: iwsfutcmd

Problem

I was recently installing PyICU on a new Ubuntu 19.04 machine that did not have pkg-config installed. The error message was as follows:

    ERROR: Command errored out with exit status 1:
     command: /home/iwsfutcmd/.virtualenvs/panlex/bin/python3 -c 'import sys, setuptools, tokenize; sys.argv[0] = '"'"'/tmp/pip-install-0x2am18z/pyicu-binary/setup.py'"'"'; __file__='"'"'/tmp/pip-install-0x2am18z/pyicu-binary/setup.py'"'"';f=getattr(tokenize, '"'"'open'"'"', open)(__file__);code=f.read().replace('"'"'\r\n'"'"', '"'"'\n'"'"');f.close();exec(compile(code, __file__, '"'"'exec'"'"'))' egg_info --egg-base pip-egg-info
         cwd: /tmp/pip-install-0x2am18z/pyicu-binary/
    Complete output (32 lines):
    Traceback (most recent call last):
      File "/tmp/pip-install-0x2am18z/pyicu-binary/setup.py", line 43, in <module>
        ICU_VERSION = os.environ['ICU_VERSION']
      File "/home/iwsfutcmd/.virtualenvs/panlex/lib/python3.7/os.py", line 678, in __getitem__
        raise KeyError(key) from None
    KeyError: 'ICU_VERSION'
    
    During handling of the above exception, another exception occurred:
    
    Traceback (most recent call last):
      File "/tmp/pip-install-0x2am18z/pyicu-binary/setup.py", line 46, in <module>
        ICU_VERSION = check_output(('icu-config', '--version')).strip()
      File "/usr/lib/python3.7/subprocess.py", line 395, in check_output
        **kwargs).stdout
      File "/usr/lib/python3.7/subprocess.py", line 472, in run
        with Popen(*popenargs, **kwargs) as process:
      File "/usr/lib/python3.7/subprocess.py", line 775, in __init__
        restore_signals, start_new_session)
      File "/usr/lib/python3.7/subprocess.py", line 1522, in _execute_child
        raise child_exception_type(errno_num, err_msg, err_filename)
    FileNotFoundError: [Errno 2] No such file or directory: 'icu-config': 'icu-config'
    
    During handling of the above exception, another exception occurred:
    
    Traceback (most recent call last):
      File "<string>", line 1, in <module>
      File "/tmp/pip-install-0x2am18z/pyicu-binary/setup.py", line 53, in <module>
        ''')
    RuntimeError:
    Please set the ICU_VERSION environment variable to the version of
    ICU you have installed.
    
    ----------------------------------------
ERROR: Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.

This implies that either ICU_VERSION needs to be set, or icu-config is missing. Being that icu-config is deprecated, and it seems strange to have to set an environment variable to install a Python package (especially as I hadn't needed to before), this had stumped me until I looked into setup.py and realized it was due to the machine not having pkg-config installed.

Solution

Change the error message to:

Please install pkg-config on your system or set the ICU_VERSION environment variable to the version of ICU you have installed.