configure: s/--with-llvm-shared-libs/--enable-llvm-shared-libs/

`--enable-llvm-shared-libs` option was recently renamed as
`--with-llvm-shared-libs`, but several error messages still mention the
old option, causing confusing.

Trivial.

configure.ac

@@ -611,7 +611,7 @@ AC_ARG_ENABLE([omx],
 AC_ARG_ENABLE([opencl],
    [AS_HELP_STRING([--enable-opencl],
          [enable OpenCL library NOTE: Enabling this option will also enable
-          --with-llvm-shared-libs
+          --enable-llvm-shared-libs
           @<:@default=no@:>@])],
    [],
    [enable_opencl=no])
@@ -1888,7 +1888,7 @@ if test "x$MESA_LLVM" != x0; then
 	can use the --with-llvm-prefix= configure flag to specify this directory.
 	NOTE: Mesa is attempting to use llvm shared libraries because you have
 	passed one of the following options to configure:
-		--with-llvm-shared-libs
+		--enable-llvm-shared-libs
 		--enable-opencl
 	If you do not want to build with llvm shared libraries and instead want to
 	use llvm static libraries then remove these options from your configure