Configure: disabled gcc atomics with Sun C (ticket #1261).

Oracle Developer Studio 12.5 introduced GCC-compatible __sync builtins.
Unfortunately, these builtins are neither GCC-compatible (they generate
warnings when used with volatile), nor working (unexpectedly fail on
unpredictable combinations of code layout and compiler flags).  As such,
the gcc builtin atomic operations configure test explicitly disabled when
compiling with Sun C.

auto/cc/conf

@@ -178,21 +178,25 @@ if [ "$NGX_PLATFORM" != win32 ]; then
     fi
 
 
-    ngx_feature="gcc builtin atomic operations"
-    ngx_feature_name=NGX_HAVE_GCC_ATOMIC
-    ngx_feature_run=yes
-    ngx_feature_incs=
-    ngx_feature_path=
-    ngx_feature_libs=
-    ngx_feature_test="long  n = 0;
-                      if (!__sync_bool_compare_and_swap(&n, 0, 1))
-                          return 1;
-                      if (__sync_fetch_and_add(&n, 1) != 1)
-                          return 1;
-                      if (n != 2)
-                          return 1;
-                      __sync_synchronize();"
-    . auto/feature
+    if [ "$NGX_CC_NAME" = "sunc" ]; then
+        echo "checking for gcc builtin atomic operations ... disabled"
+    else
+        ngx_feature="gcc builtin atomic operations"
+        ngx_feature_name=NGX_HAVE_GCC_ATOMIC
+        ngx_feature_run=yes
+        ngx_feature_incs=
+        ngx_feature_path=
+        ngx_feature_libs=
+        ngx_feature_test="long  n = 0;
+                          if (!__sync_bool_compare_and_swap(&n, 0, 1))
+                              return 1;
+                          if (__sync_fetch_and_add(&n, 1) != 1)
+                              return 1;
+                          if (n != 2)
+                              return 1;
+                          __sync_synchronize();"
+        . auto/feature
+    fi
 
 
     if [ "$NGX_CC_NAME" = "ccc" ]; then