Move closer to the standard autotools idiom.

[dag-erling]

• Create src/config.h instead of passing everything on the compiler command line.

• To reduce the magnitude of this change, add --include src/config.h to CFLAGS instead of adding #include "src/config.h" at the top of each source file. Not all compilers support this, but I don't think we care about anything other than gcc and clang at this point.

• Rather than generate src/config_opts.inc, emit JQ_CONFIG directly into src/config.h.

• Rather than generate src/version.h, define JQ_VERSION as an alias for the standard PACKAGE_VERSION.