implement BORG_LIBC env variable, fixes #5870

docs/usage/general/environment.rst.inc

@@ -68,6 +68,11 @@ General:
         When set to no (default: yes), system information (like OS, Python version, ...) in
         exceptions is not shown.
         Please only use for good reasons as it makes issues harder to analyze.
+    BORG_LIBC
+        borg uses ``ctypes.util.find_library`` to locate the 'c' library (aka libc).
+        find_library needs a shell and will invoke some tools like ldconfig, gcc/cc or objdump.
+        If a shell or these tools are not available, you can give the name of your libc via
+        BORG_LIBC=libc.so.6 (for example) and borg will not try the ``find_library`` call.
     BORG_SELFTEST
         This can be used to influence borg's builtin self-tests. The default is to execute the tests
         at the beginning of each borg command invocation.

src/borg/xattr.py

@@ -62,7 +62,7 @@ def get_all(path, follow_symlinks=True):
             return {}
 
 
-libc_name = find_library('c')
+libc_name = os.environ.get('BORG_LIBC') or find_library('c')
 if libc_name is None:
     # find_library didn't work, maybe we are on some minimal system that misses essential
     # tools used by find_library, like ldconfig, gcc/cc, objdump.