Allow library path to be specified in env

This helps people who have this installed in non-standard paths (e.g.,
for a Nix environment). The default here only seems valid for a very
typical autoconf setup with PREFIX=/usr/local.

pylibhackrf.py

@@ -13,7 +13,7 @@ logging.basicConfig()
 logger = logging.getLogger('HackRf Core')
 logger.setLevel(logging.DEBUG)
 
-libhackrf = CDLL('/usr/local/lib/libhackrf.so')
+libhackrf = CDLL(os.environ.get('LIBHACKRF', '/usr/local/lib/libhackrf.so'))
 
 def enum(*sequential, **named):
     enums = dict(zip(sequential, range(len(sequential))), **named)
@@ -535,4 +535,4 @@ class HackRf(object):
         bytes2 = bytes[0:size * 2]
         iq.real, iq.imag = bytes2[::2], bytes2[1::2]
         iq /= 128.0
-        return iq
+        return iq