setup.py: use PKG_CONFIG env var to get the pkg-config to use

pkg-config may be prefixed when cross-compiling

setup.py

@@ -72,16 +72,17 @@ def get_config_from_pkg_config():
     """
     Get distutils-compatible extension arguments using pkg-config.
     """
+    pkg_config = os.environ.get('PKG_CONFIG', 'pkg-config')
     try:
         raw_cflags = subprocess.check_output(
-            ["pkg-config", "--cflags", "--libs"]
+            [pkg_config, "--cflags", "--libs"]
             + ["lib" + name for name in FFMPEG_LIBRARIES]
         )
     except FileNotFoundError:
-        print("pkg-config is required for building PyAV")
+        print(f"{pkg_config} is required for building PyAV")
         exit(1)
     except subprocess.CalledProcessError:
-        print("pkg-config could not find libraries {}".format(FFMPEG_LIBRARIES))
+        print(f"{pkg_config} could not find libraries {FFMPEG_LIBRARIES}")
         exit(1)
 
     known, unknown = parse_cflags(raw_cflags.decode("utf-8"))