Revert "adds -latomic to. ghc-prim"

This commit was never properly justified and relies on the existence of
libatomic, which doesn't appear to exist on Darwin.

This reverts commit ec9aacf.

aclocal.m4

@@ -1296,6 +1296,24 @@ AC_SUBST(GccIsClang)
 rm -f conftest.txt
 ])
 
+# FP_GCC_SUPPORTS__ATOMICS
+# ------------------------
+# Does gcc support the __atomic_* family of builtins?
+AC_DEFUN([FP_GCC_SUPPORTS__ATOMICS],
+[
+   AC_REQUIRE([AC_PROG_CC])
+   AC_MSG_CHECKING([whether GCC supports __atomic_ builtins])
+   echo 'int test(int *x) { int y; __atomic_load(&x, &y, __ATOMIC_SEQ_CST); return x; }' > conftest.c
+   if $CC -c conftest.c > /dev/null 2>&1; then
+       CONF_GCC_SUPPORTS__ATOMICS=YES
+       AC_MSG_RESULT([yes])
+   else
+       CONF_GCC_SUPPORTS__ATOMICS=NO
+       AC_MSG_RESULT([no])
+   fi
+   rm -f conftest.c conftest.o
+])
+
 # FP_GCC_SUPPORTS_NO_PIE
 # ----------------------
 # Does gcc support the -no-pie option? If so we should pass it to gcc when

configure.ac

@@ -739,6 +739,11 @@ FP_GCC_VERSION
 dnl ** See whether gcc supports -no-pie
 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?])
+
 FP_GCC_EXTRA_FLAGS
 
 dnl ** look to see if we have a C compiler using an llvm back end.

libraries/ghc-prim/Setup.hs

@@ -18,7 +18,7 @@ import System.Exit
 import System.Directory
 
 main :: IO ()
-main = do let hooks = autoconfUserHooks {
+main = do let hooks = simpleUserHooks {
                   regHook = addPrimModule
                           $ regHook simpleUserHooks,
                   buildHook = build_primitive_sources