Various heap profiling improvements.

Add the --disable-prof-libgcc configure option, and add backtracing
based on libgcc, which is used by default.

Fix a bug in hash().

Fix various configuration-dependent compilation errors.

jemalloc/INSTALL

@@ -48,12 +48,16 @@ any of the following arguments (not a definitive list) to 'configure':
     Enable heap profiling and leak detection functionality.  Use the 'B', 'F',
     'I', 'L', and 'U' options to control these features.
 
+--disable-prof-libgcc
+    Disable the use of libgcc's backtracing functionality.  Ordinarily, libgcc's
+    backtracing functionality is superior to the alternatives, but it may fail
+    to capture backtraces on some systems.
+
 --enable-prof-libunwind
     Use the libunwind library (http://www.nongnu.org/libunwind/) for stack
-    backtracing, rather than frame pointers.  libunwind is quite slow in
-    comparison to frame pointer-based backtracing, but it has the advantage of
-    working on applications/libraries that were compiled with
-    -fomit-frame-pointer.
+    backtracing.  libunwind is quite slow, but it tends to work across a wider
+    variety of system configurations than the default backtracing code, which is
+    based on libgcc functionality or gcc intrinsics.
 
 --disable-tiny
     Disable tiny (sub-quantum-sized) object support.  Technically it is not