configure.ac/meson.build: Fix -latomic test

When compiling with LLVM 6.0 on x86 (32-bit) for Android, the test
fails to detect that -latomic is actually required, as the atomic
call is inlined.

In the code itself (src/util/disk_cache.c), we see this pattern:
p_atomic_add(cache->size, - (uint64_t)size);
where cache->size is an uint64_t *, and results in the following
link time error without -latomic:
src/util/disk_cache.c:628: error: undefined reference to '__atomic_fetch_add_8'

Fix the configure/meson test to replicate this pattern, which then
correctly realizes the need for -latomic.

Reviewed-by: Matt Turner <[email protected]>
Signed-off-by: Nicolas Boichat <[email protected]>

1 files changed, 4 insertions, 2 deletions

diff --git a/meson.build b/meson.build
index 6e617668096..e52b4a51093 100644
--- a/meson.build
+++ b/meson.build
@@ -849,8 +849,10 @@ if cc.compiles('int main() { int n; return __atomic_load_n(&n, __ATOMIC_ACQUIRE)
   # as ARM.
   if not cc.links('''#include <stdint.h>
                      int main() {
-                       uint64_t n;
-                       return (int)__atomic_load_n(&n, __ATOMIC_ACQUIRE);
+                       struct {
+                         uint64_t *v;
+                       } x;
+                       return (int)__atomic_load_n(x.v, __ATOMIC_ACQUIRE);
                      }''',
                   name : 'GCC atomic builtins required -latomic')
     dep_atomic = cc.find_library('atomic')