Support environmental variables

[Arfrever]

It would be useful if Mozc supported environmental variables for additional run-time configurability. Suggested environmental variables:

• MOZC_SERVER_DIRECTORY This variable would override build-time server_dir GYP option (/usr/lib/mozc by default).
• MOZC_DOCUMENTS_DIRECTORY This variable would override build-time document_dir GYP option (/usr/lib/mozc/documents by default).
• MOZC_ZINNIA_MODEL_FILE This variable would override build-time zinnia_model_file GYP option (e.g. /usr/share/tegaki/models/zinnia/handwriting-ja.model).
• MOZC_CONFIGURATION_DIRECTORY This variable would set Mozc configuration directory (e.g. /home/username/.mozc).
• HOME This variable would be used on Unix-like systems for calculating Mozc configuration directory if MOZC_CONFIGURATION_DIRECTORY variable is not set. If MOZC_CONFIGURATION_DIRECTORY variable is not set and HOME variable is set, then ${HOME}/.mozc would be used. If neither MOZC_CONFIGURATION_DIRECTORY variable nor HOME variable is set, then preexisting logic would be used, which gets home directory from /etc/passwd file.

Patch: mozc-environmental_variables.patch.txt

[hiroyuki-komatsu]

Hi, thank you for the suggestions.

We do not have a plan to support run-time configurations of the install paths, because it introduces additional edge cases we need to care about.

Instead, we have supported $XDG_CONFIG_HOME as mentioned in #474