configure.ac: check cmake availability

configure.ac: check cmake availability

In the configure.ac file, detect if a cmake executable is
available. Two new variables are now available in any
Makefile.am file: HAVE_CXX (which tells if a C++ compiler
is available) and HAVE_CMAKE (which indicates whether a cmake
executable is available).

Acked-by: Philippe Proulx <eeppeliteloop@gmail.com>
Acked-by: Sebastien Boisvert <sboisvert@gydle.com>
Signed-off-by: Sebastien Boisvert <sboisvert@gydle.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>