definition and documentation of environmental variable defining configuration file path

[adokter]

The following should be documented in the example options.conf file in /etc:

vol2bird/lib/constants.h

Lines 164 to 167 in 4420cb1

	// name of (optional) environmental variable containing path to user configuration file
	#define OPTIONS_CONF "OPTIONS_CONF"
	// default user configuration file name to search for in working directory
	#define OPTIONS_FILE "options.conf"

Also:

• the feature of specifying this environmental variable should be described in the general vol2bird documentation
• OPTIONS_CONF contains the directory path, it would be better to make this a file path to the configuration file instead.