Enable C++17 language features to satisfy libsigc++ requirements

[amotl]

Dear Uwe, Bert, Gerhard and all other contributors,

first things first: Thanks a stack for conceiving and maintaining libsigrok.

While working on sigrokproject/sigrok-meter#1, when trying to build on macOS Catalina with recent compilers (GCC 10.2.0) and libraries (libsigc++ 3.0.6) installed through Homebrew, the process croaked with error: 'invoke' is not a member of 'std'.

It turned out that libsigc++ is telling us [1]:

Compatible compilers must support C++17, such as the decltype(auto) specifier (from C++14) and std::invoke().

So, this brings in C++17 support by updating the ax_cxx_compile_stdcxx.m4 macro to the most recent version from [2] and by adjusting some bits in configure.ac. Please bear with me when I might have done something wrong, as C/C++ is not my native habitat. However, with those changes, GCC stopped croaking on my end.

With kind regards,
Andreas.

P.S.: Others reading here also aiming at building on a recent macOS, please also note #123.

[1] https://github.com/libsigcplusplus/libsigcplusplus#compatibility
[2] http://git.savannah.gnu.org/gitweb/?p=autoconf-archive.git;a=blob_plain;f=m4/ax_cxx_compile_stdcxx.m4

[depili]

This might break the compilation on other hosts that don't have C++17, but have C++11.

[depili]

This has now been implemented in cbcc060

Could help if you closed the PR

[amotl]

Dear @depili and @gsigh,

thanks a stack!

With kind regards,
Andreas.