Fix how to call make for build dir different from source one.
all`
You can also build MIR in a separate directory. In this case you
- need to use `make SRC_DIR=<path to MIR sources>`. **All other calls
- of make** should have the same `SRC_DIR` value on the make command
- line.
+need to use `make SRC_DIR=<path to MIR sources> -f <path to MIR sources>/GNUmakefile`.
+**All other calls of make** should have the
+same `SRC_DIR` value and `-f` argument on the make command line.
By default MIR is built in release mode (with optimizations). If you
want to build debugging version (without optimizations and additional
