Reverse the cc-silence default.

Replace --enable-cc-silence with --disable-cc-silence, so that by
default people won't see spurious warnings when building jemalloc.
diff --git a/INSTALL b/INSTALL
index 8530643..55c919a 100644
--- a/INSTALL
+++ b/INSTALL
@@ -71,10 +71,10 @@
     versions of jemalloc can coexist in the same installation directory.  For
     example, libjemalloc.so.0 becomes libjemalloc<suffix>.so.0.
 
---enable-cc-silence
-    Enable code that silences non-useful compiler warnings.  This is helpful
-    when trying to tell serious warnings from those due to compiler
-    limitations, but it potentially incurs a performance penalty.
+--disable-cc-silence
+    Disable code that silences non-useful compiler warnings.  This is mainly
+    useful during development when auditing the set of warnings that are being
+    silenced.
 
 --enable-debug
     Enable assertions and validation code.  This incurs a substantial
diff --git a/configure.ac b/configure.ac
index 04cefe9..b47d572 100644
--- a/configure.ac
+++ b/configure.ac
@@ -577,18 +577,17 @@
 cfghdrs_tup="${cfghdrs_tup} include/jemalloc/internal/jemalloc_internal_defs.h:${srcroot}include/jemalloc/internal/jemalloc_internal_defs.h.in"
 cfghdrs_tup="${cfghdrs_tup} test/include/test/jemalloc_test_defs.h:${srcroot}test/include/test/jemalloc_test_defs.h.in"
 
-dnl Do not silence irrelevant compiler warnings by default, since enabling this
-dnl option incurs a performance penalty.
+dnl Silence irrelevant compiler warnings by default.
 AC_ARG_ENABLE([cc-silence],
-  [AS_HELP_STRING([--enable-cc-silence],
-                  [Silence irrelevant compiler warnings])],
+  [AS_HELP_STRING([--disable-cc-silence],
+                  [Do not silence irrelevant compiler warnings])],
 [if test "x$enable_cc_silence" = "xno" ; then
   enable_cc_silence="0"
 else
   enable_cc_silence="1"
 fi
 ],
-[enable_cc_silence="0"]
+[enable_cc_silence="1"]
 )
 if test "x$enable_cc_silence" = "x1" ; then
   AC_DEFINE([JEMALLOC_CC_SILENCE], [ ])