Fix missing PREFIX var for make install

Signed-off-by: falkTX <falktx@falktx.com>

Makefile

@@ -6,8 +6,17 @@
 
 include dpf/Makefile.base.mk
 
+# --------------------------------------------------------------
+# Build targets
+
 all: carla dgl plugins gen
 
+# --------------------------------------------------------------
+# Build config
+
+PREFIX  ?= /usr/local
+DESTDIR ?=
+
 # --------------------------------------------------------------
 # Carla config