fix: invoke MKDIR_P before changing directories

fix: invoke MKDIR_P before changing directories

In autoconf < 2.72d `AC_PROG_MKDIR_P` may fall back to using
`install-sh` and that may be referenced as a relative path.

To avoid issues with relative paths causing the command to not be
found, the build directories are created before changing the working
directory.

One way to to test the behaviour prior to this commit is to configure
the build similar to the following:

    ./configure MKDIR_P="$(realpath --relative-to="$(pwd)" \
    $(command -v mkdir))" BUILD_EXAMPLES_FROM_TREE=1

Fixes https://bugs.lttng.org/issues/1404

Change-Id: I2d66254cd8c208f9236d55c6ef1b83c580560c7c
Signed-off-by: Kienan Stewart <kstewart@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>