configure: Fix HAVE_C11_ATOMICS macro

Previously we were using AC_DEFINE instead of
AC_DEFINE_UNQUOTED, resulted in the variable not being
interpolated.

Fixes #17505.

(cherry picked from commit 2c66f364)

configure.ac

@@ -715,7 +715,9 @@ FP_GCC_SUPPORTS_NO_PIE
 dnl ** Used to determine how to compile ghc-prim's atomics.c, used by
 dnl    unregisterised, Sparc, and PPC backends.
 FP_GCC_SUPPORTS__ATOMICS
-AC_DEFINE([HAVE_C11_ATOMICS], [$CONF_GCC_SUPPORTS__ATOMICS], [Does GCC support __atomic primitives?])
+if test $CONF_GCC_SUPPORTS__ATOMICS = YES ; then
+  AC_DEFINE([HAVE_C11_ATOMICS], [1], [Does GCC support __atomic primitives?])
+fi
 
 FP_GCC_EXTRA_FLAGS