Configure error on ppc64le GNU/Linux

I want to build `boost +mpi %gcc @10.2.0` via Spack https://github.com/spack/spack on Summit, which is a ppc64le GNU/Linux system. This fails due to a configure error in Boost.

Although Spack calls Boost's `bootstrap.sh` as expected with `--with-toolset=gcc`, `bootstrap.sh` tries to use the `xlcc` toolset internally. The problem is that `bootstrap.sh` does not pass the `TOOLSET` variable as argument to the `build.sh` script.

This patch solves the problem for me:

```diff
diff -u bootstrap.sh.orig bootstrap.sh
--- a/bootstrap.sh.orig 2020-12-03 00:00:59.000000000 -0500
+++ a/bootstrap.sh 2021-01-08 13:38:30.000000000 -0500
@@ -223,7 +223,7 @@
if test "x$BJAM" = x; then
$ECHO "Building B2 engine.."
pwd=`pwd`
- (cd "$my_dir/tools/build/src/engine" && ./build.sh)
+ (cd "$my_dir/tools/build/src/engine" && ./build.sh "$TOOLSET")
if [ $? -ne 0 ]; then
echo
echo "Failed to build B2 build engine"
```