Accept that glibc's statx() falls back for us

So we don't need to have a high kernel requirement on its account.

I needed to introduce a configure-time check because we need to include
a header to get the __GLIBC__ macro, but we can't include any header in
assembler until we know it's glibc (we need to know that the header is
assembler-safe). glibc, uClibc and MUSL do provide an assembler-safe
features.h, but Bionic does not.

And we need to know that it's glibc's implementation, since the fallback
was not required. The other three libraries may not implement such a
thing when they get around to adding the system call.

Fixes: QTBUG-74526
Change-Id: I1004b4b819774c4c9296fffd158d14da98bf571c
Reviewed-by: Fabian Vogt <fabian@ritter-vogt.de>
Reviewed-by: Simon Hausmann <simon.hausmann@qt.io>

src/corelib/configure.json

@@ -374,6 +374,16 @@
                 ]
             }
         },
+        "glibc": {
+            "label": "GNU libc",
+            "type": "compile",
+            "test": {
+                "include": "stdlib.h",
+                "main": [
+                    "return __GLIBC__;"
+                ]
+            }
+        },
         "inotify": {
             "label": "inotify",
             "type": "compile",
@@ -593,6 +603,12 @@
             "condition": "libs.glib",
             "output": [ "privateFeature", "feature" ]
         },
+        "glibc": {
+            "label": "GNU libc",
+            "autoDetect": "config.linux",
+            "condition": "tests.glibc",
+            "output": [ "privateFeature" ]
+        },
         "iconv": {
             "label": "iconv",
             "purpose": "Provides internationalization on Unix.",

src/corelib/global/minimum-linux_p.h

@@ -78,7 +78,11 @@ QT_BEGIN_NAMESPACE
  * - statx                      4.11                    QT_CONFIG(statx)
  */
 
-#if QT_CONFIG(statx)
+#if QT_CONFIG(statx) && !QT_CONFIG(glibc)
+// if using glibc, the statx() function in sysdeps/unix/sysv/linux/statx.c
+// falls back to stat() for us.
+// (Using QT_CONFIG(glibc) instead of __GLIBC__ because the macros aren't
+// defined in assembler mode)
 #  define MINLINUX_MAJOR        4
 #  define MINLINUX_MINOR        11
 #  define MINLINUX_PATCH        0