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This uses the AX_CXX_COMPILE_STDCXX() instead of
AX_CXX_COMPILE_STDCXX_1{4,7}() so that the error messages we generate
make slightly more sense.

We get errors like:
configure.ac:24: error: invalid first argument `20' to AX_CXX_COMPILE_STDCXX

Instead of:
./configure: line 5132: syntax error near unexpected token `noext'
./configure: line 5132: `AX_CXX_COMPILE_STDCXX_20(noext)'

Tested:
    Builds and passes unit tests.

Change-Id: I22f71b4f83fec0507de1175ac656ce63e9acecea
Signed-off-by: William A. Kennington III <wak@google.com>