Add the possibility to set globally INSTALL_DIR

[ycollet]

Add the possibility to set globally INSTALL_DIR.
Currently, INSTALL_DIR is set to: INSTALL_DIR = /usr/lib/lv2
On Fedora / Red Hat, the INSTALL_DIR should be set to: INSTALL_DIR = /usr/lib64/lv2

What about adding:

INSTALL_DIR ?= /usr/lib/lv2

In each Makefile ?

[Hermann]

Hi ycollet

You could simply overwrite the INSTALL_DIR variable, there is no need for the ? to do so.
Just try
make install INSTALL_DIR='/usr/lib64/lv2'