package/matchbox-starup-monitor: fix build without C++

matchbox-startup-monitor is an ageing package, and uses an old
configure.ac with archaic constructs. This had generated a configure
script that incorrectly tries to look for and validate a C++ compiler:

    checking for powerpc64le-buildroot-linux-gnu-g++... no
    checking whether we are using the GNU C++ compiler... no
    checking whether no accepts -g... no
    checking dependency style of no... none
    checking how to run the C++ preprocessor... /lib/cpp
    configure: error: C++ preprocessor "/lib/cpp" fails sanity check

Calling autoreconf fixes the issue, as the generated configure no longer
tries to look for a C++ compiler at all anymore. Running autoreconf does
not add any new dependency, as they are already in the dependency chain
via other packages.

Fixes:
    http://autobuild.buildroot.org/results/223/223f43dd76ee907c5f25c4fee94a0f5d75614dd5/



See also similar changes:
    9993a36f package/pamtester: fix build without C++
    c05cc5de package/madplay: needs autoreconf
    eae18d01 libmad: needs autoreconf
    43274dd3 package/libid3tag: needs autoreconf

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>