fix over-zealous detection of libatomic (it was picked up at configure
time if present at all, rather than only if needed)

Index: meson.build
--- meson.build.orig
+++ meson.build
@@ -720,7 +720,19 @@ if features['sdl2-gamepad']
     sources += files('input/sdl_gamepad.c')
 endif
 
-stdatomic_dep = cc.find_library('atomic', required: false)
+if cc.links('''
+#include <stdint.h>
+int main(void) {
+  uint32_t x32 = 0;
+  uint64_t x64 = 0;
+  __atomic_load_n(&x32, __ATOMIC_SEQ_CST);
+  __atomic_load_n(&x64, __ATOMIC_SEQ_CST);
+  return 0;
+}''', name : 'built-in atomics')
+    stdatomic_dep = dependency('', required : false)
+else
+    stdatomic_dep = cc.find_library('atomic')
+endif
 features += {'stdatomic': cc.has_header_symbol('stdatomic.h', 'atomic_int', dependencies: stdatomic_dep,
                                                required: get_option('stdatomic'))}
 if features['stdatomic']