PREFIX not settable per env var

[fps]

This might help:

diff --git a/Makefile b/Makefile
index 81fb3a3..90ed65a 100644
--- a/Makefile
+++ b/Makefile
@@ -20,7 +20,9 @@ ifeq ($(MOD),1)
 EFFECTS_DIR += HardwareBypass
 endif

-INSTALL_PATH = /usr/local/lib/lv2
+PREFIX ?= /usr
+
+INSTALL_PATH = $(PREFIX)/lib/lv2

 all:
                @for fx in $(EFFECTS_DIR); do \