gst-env: Don't set DYLD_LIBRARY_PATH on macOS

This is not actually needed because everything we build is using
@rpath already, and setting it causes dynamic linker path priority
issues with macOS internals causing *all* programs to fail to run
inside gst-env:

```
$ vim
dyld: Symbol not found: __cg_jpeg_resync_to_restart
  Referenced from: /System/Library/Frameworks/ImageIO.framework/Versions/A/Resources/libTIFF.dylib
  Expected in: /Users/nirbheek/projects/repos/gst-build/_build_macos/subprojects/libjpeg-turbo-2.1.0/libJPEG.dylib
 in /System/Library/Frameworks/ImageIO.framework/Versions/A/Resources/libTIFF.dylib
Abort trap: 6
```

In this case it is caused by libjpeg.dylib, but it can happen with
other dylibs that conflict with dylibs used by macOS internally.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-build/-/merge_requests/257>

diff --git a/gst-env.py b/gst-env.py
index 40d66e3..df3e402 100755 (executable)
--- a/gst-env.py
+++ b/gst-env.py
@@ -72,6 +72,8 @@ def stringify(o):
     raise AssertionError('Object {!r} must be a string or a list'.format(o))
 
 def prepend_env_var(env, var, value, sysroot):
+    if var is None:
+        return
     if value.startswith(sysroot):
         value = value[len(sysroot):]
     # Try not to exceed maximum length limits for env vars on Windows
@@ -275,7 +277,8 @@ def get_subprocess_env(options, gst_version):
     if os.name == 'nt':
         lib_path_envvar = 'PATH'
     elif platform.system() == 'Darwin':
-        lib_path_envvar = 'DYLD_LIBRARY_PATH'
+        # RPATH is sufficient on macOS, and DYLD_LIBRARY_PATH can cause issues with dynamic linker path priority
+        lib_path_envvar = None
     else:
         lib_path_envvar = 'LD_LIBRARY_PATH'