Set AC_LANG to C++
Set AC_LANG to C++

And move compiler configuration prior to test macros, otherwise
c++14 header tests fail.

Signed-off-by: Brad Bishop <bradleyb@fuzziesquirrel.com>

Change-Id: I64a7f3d22bb655c06a8ab845e636f27a9cc17c33

openbmc/phosphor-inventory-manager,openbmc/phosphor-inventory-manager