Add warnings when compiling from source

setup.py

@@ -6,6 +6,31 @@
 import shlex
 import subprocess
 import sys
+from time import sleep
+
+
+def is_virtualenv():
+    return sys.base_prefix != sys.prefix
+
+
+if platform.system() == "Darwin":
+    major_version = int(platform.mac_ver()[0].split(".")[0])
+    if major_version < 12:
+        print(
+            "\033[1;91mWarning!\033[0m You are using an EOL, unsupported, and out-of-date OS."
+        )
+        sleep(3)
+
+# Don't show message when using our project tooling.
+if not is_virtualenv() or os.getenv("_PYAV_ACTIVATED", "") != "1":
+    print(
+        "\n\033[1;91mWarning!\033[0m You are installing from source.\n"
+        "It is \033[1;37mEXPECTED\033[0m that it will fail. You are \033[1;37mREQUIRED\033[0m"
+        " to use ffmpeg 7.\nYou \033[1;37mMUST\033[0m have Cython, pkg-config, and a C compiler.\n"
+    )
+    if os.getenv("GITHUB_ACTIONS") != "true":
+        sleep(3)
+
 
 from Cython.Build import cythonize
 from Cython.Compiler.AutoDocTransforms import EmbedSignature
@@ -141,7 +166,7 @@ def parse_cflags(raw_flags):
     cythonize = lambda ext, **kwargs: [ext]
     use_pkg_config = False
 
-# Locate FFmpeg libraries and headers.
+# Locate ffmpeg libraries and headers.
 if FFMPEG_DIR is not None:
     extension_extra = get_config_from_directory(FFMPEG_DIR)
 elif use_pkg_config: