configure: Update from c++14 to c++17

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>

configure.ac

@@ -21,7 +21,7 @@ AS_IF([test "$HAVE_UAPI_LINUX_LPC_CTRL_H" != ""],
 )
 
 # Checks for typedefs, structures, and compiler characteristics.
-AX_CXX_COMPILE_STDCXX_14([noext])
+AX_CXX_COMPILE_STDCXX(17, [noext], [mandatory])
 AX_APPEND_COMPILE_FLAGS([-fpic -Wall], [CFLAGS])
 AX_APPEND_COMPILE_FLAGS([-fpic -Wall], [CXXFLAGS])