Nicer cxx11 error message pass from bootstrap to build.

Expanded error message for cxx11 requirment. And now passing bootstrap
arguments to build script to allow choosing toolset from bootstrap also.

bootstrap.sh

@@ -10,7 +10,7 @@
 # Build b2
 echo "Building the B2 engine.."
 pwd=`pwd`
-( cd "./src/engine" && ./build.sh )
+( cd "./src/engine" && ./build.sh "$*" )
 if [ $? -ne 0 ]; then
     echo
     echo "Failed to build the B2 engine." 1>&2

src/engine/build.sh

@@ -194,6 +194,10 @@ if ! test_cxx11 $B2_TOOLSET ; then
     error_exit "
 A C++11 capable compiler is required for building the B2 engine.
 Toolset '$B2_TOOLSET' does not appear to support C++11.
+
+** Note, the C++11 capable compiler is _only_ required for building the B2
+** engine. The B2 build system allows for using any C++ level and any other
+** supported language and resource in your projects.
 "
 fi