configure.ac, Makefile.am: introduce THRIFT variable to support cross-compilation

The thrift build system currently assumes that the thrift compiler is
always available in $(top_builddir)/compiler/cpp/thrift. However, in a
cross-compilation context, this location contains the thrift compiler
built for the target... which obviously will not run on the build
machine.

In order to support such cross-compilation situation, we introduce the
THRIFT variable as a an argument for the configure script (using
AC_ARG_VAR). If not specified, it defaults to the existing value of
using compiler/cpp/thrift from the build directory, but it can be
overridden when calling ./configure.

Note that $(top_builddir) cannot be used within the configure script,
so we simply use `pwd`, which is the same as the top_builddir.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

This closes #1336
This closes #1350