Allow override of runtime search path

[salsaman]

In the frei0r spec, a search list of plugin locations is provided:

 http://www.piksel.org/frei0r/1.1/spec/group__pluglocations.html

However, this will fail for example on 64 bit systems if the frei0r plugins are installed in /usr/lib64/frei0r-1/.

To get around this, I suggest allowing use of an environment variable: FREI0R_PLUGIN_DIR which would override the second directory path (/usr/local/lib/frei0r-1/) if present.

[jaromil]

This has to do with the host application, not with frei0r plugins.

FWIW FreeJ searches in the following paths:

addsearchdir("/usr/lib/frei0r-1");

addsearchdir("/usr/lib64/frei0r-1");

addsearchdir("/usr/local/lib/frei0r-1");

addsearchdir("/opt/local/lib/frei0r-1");

[jaromil]

pkg-config can solve the problem: we already provide a frei0r.pc that is generated at compile time and installed in PKG_CONFIG_DIR, that can be used by host applications to ./configure the plugin path when they are compiled or binary packaged into a distribution.

the problem still persists:, the path in our .pc file doesn't honours the /lib64 variant on 64bit systems:

libdir=${exec_prefix}/lib

inside frei0r.pc.in should be better formed by configure.ac