Reduce dependency on ctypes when discovering glibc version.

news/6543.bugfix

@@ -0,0 +1 @@
+If possible, the version info for glibc is extracted from ``os`` instead of ``ctypes`` (which may be missing).

news/6675.bugfix

@@ -0,0 +1 @@
+If possible, the version info for glibc is extracted from ``os`` instead of ``ctypes`` (which may be missing).

src/pip/_internal/utils/glibc.py

@@ -1,6 +1,6 @@
 from __future__ import absolute_import
 
-import ctypes
+import os
 import re
 import warnings
 
@@ -14,6 +14,19 @@ def glibc_version_string():
     # type: () -> Optional[str]
     "Returns glibc version string, or None if not using glibc."
 
+    try:  # https://github.com/pypa/pip/issues/6675#issue-463147612
+        glibc_version = os.confstr("CS_GNU_LIBC_VERSION").split()
+    except (AttributeError, OSError, ValueError):
+        pass  # os.confstr() or CS_GNU_LIBC_VERSION not available...
+    else:
+        if len(glibc_version) == 2:
+            return glibc_version[-1]
+
+    try:
+        import ctypes
+    except ImportError:
+        return None
+
     # ctypes.CDLL(None) internally calls dlopen(NULL), and as the dlopen
     # manpage says, "If filename is NULL, then the returned handle is for the
     # main program". This way we can let the linker do the work to figure out