Require C++17 when building w/ gtest support

[d-torrance]

This is required by googletest since 0.17.0. When using older compilers, ./configure will fail, not because it can't find gtest, but because it runs into this:

configure:4586: checking for gtest/gtest.h
configure:4586: /opt/spack/opt/spack/linux-zen/compiler-wrapper-1.0-5ek7f4quicgccbu7qgixwxvwugqo6b6i/libexec/spack/gcc/g++ -c -g -O2  conftest.cpp >&5
In file included from /opt/spack/opt/spack/linux-zen/googletest-1.17.0-tvc36uogphcgyc6sandua5bz4y4k4rqm/include/gtest/gtest-message.h:57,
                 from /opt/spack/opt/spack/linux-zen/googletest-1.17.0-tvc36uogphcgyc6sandua5bz4y4k4rqm/include/gtest/gtest-assertion-result.h:46,
                 from /opt/spack/opt/spack/linux-zen/googletest-1.17.0-tvc36uogphcgyc6sandua5bz4y4k4rqm/include/gtest/gtest.h:63,
                 from conftest.cpp:47:
/opt/spack/opt/spack/linux-zen/googletest-1.17.0-tvc36uogphcgyc6sandua5bz4y4k4rqm/include/gtest/internal/gtest-port.h:273:2: error: #error C++ versions less than C++17 are not supported.
 #error C++ versions less than C++17 are not supported.
  ^~~~~