Configuring the timer interval in make watch

[tueda]

Currently, make watch tries to typeset every second. The interval is hardcoded:

makefile4latex/Makefile

Line 1281 in 751bd75

sleep 1; \

Though it is not a POSIX feature, the sleep command on Linux or macOS accepts a real number as the interval, e.g., 0.1. It may be useful if the user can specify the interval by a variable, let's say, sleep_interval.

[tueda]

Variable (or possibly environment variable) MAKEFILE4LATEX_WAIT_COMMAND has been added. Example:

MAKEFILE4LATEX_WAIT_COMMAND = sleep 0.1  # GNU coreutils
MAKEFILE4LATEX_WAIT_COMMAND = perl -MTime::HiRes=sleep -e sleep -e 0.1  # perl >= 5.7.3