By default, configure sets the prefix for files it installs to
/usr/local. The user of configure can select a different
prefix using the --prefix and --exec-prefix options.
There are two ways to change the default: when creating
configure, and when running it.
Some software packages might want to install in a directory other than
/usr/local by default. To accomplish that, use the
AC_PREFIX_DEFAULT macro.
Set the default installation prefix to prefix instead of /usr/local.
It may be convenient for users to have configure guess the
installation prefix from the location of a related program that they
have already installed. If you wish to do that, you can call
AC_PREFIX_PROGRAM.
If the user did not specify an installation prefix (using the
--prefix option), guess a value for it by looking for
program in PATH, the way the shell does. If program
is found, set the prefix to the parent of the directory containing
program, else default the prefix as described above
(/usr/local or AC_PREFIX_DEFAULT). For example, if
program is gcc and the PATH contains
/usr/local/gnu/bin/gcc, set the prefix to /usr/local/gnu.