android: Fix file system library usage on gcc 7 and 8

On gcc versions older than 9, the file system library, used by the
Android camera HAL configuration file parser, is implemented in a
separate static library. Furthermore, on gcc 7, it's provided in the
std::experimental namespace. This breaks compilation of the HAL on gcc
7, and linking on gcc 8.

Fix the compilation issue by conditionally including
<experimental/filesystem> and creating a namespace alias in std, and the
link issue by linking to libstdc++fs on gcc versions older than 9.

The inclusion of <experimental/filesystem> is a bit of a hack, and when
we'll start using the file system library in another compilation unit,
we should then move all this to an internal helper to abstract the
compiler version.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Hirokazu Honda <hiroh@chromium.org>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>

meson.build

@@ -76,6 +76,14 @@ if cc.get_id() == 'gcc'
         error('gcc version is too old, libcamera requires 7.0 or newer')
     endif
 
+    # On gcc 7 and 8, the file system library is provided in a separate static
+    # library.
+    if cc.version().version_compare('<9')
+        cpp_arguments += [
+            '-lstdc++fs',
+        ]
+    endif
+
     # gcc 7.1 introduced processor-specific ABI breakages related to parameter
     # passing on ARM platforms. This generates a large number of messages
     # during compilation with gcc >=7.1. Silence them.

src/android/camera_hal_config.cpp

@@ -6,7 +6,14 @@
  */
 #include "camera_hal_config.h"
 
+#if defined(_GLIBCXX_RELEASE) && _GLIBCXX_RELEASE < 8
+#include <experimental/filesystem>
+namespace std {
+namespace filesystem = std::experimental::filesystem;
+}
+#else
 #include <filesystem>
+#endif
 #include <stdio.h>
 #include <stdlib.h>
 #include <string>