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.

[itchyny]

This is a nice improvement, thank you.

[itchyny]

@dag-erling
Sorry, I didn't check the behavior changes of updating version.h before merging. The version string should be followed by the git commit hash, or -dirty suffix if any of the source code is changed. So if I checkout different branch and then make, (or even if I edit one line of main.c and then make), the version string should change. But after merging your PR, it seems that the version string is cached and even ./configure doesn't change the version. This is critical for maintainers because we can't trust the version string of the issue reports. Can you fix this problem?

[itchyny]

Can we use CONFIGURE_DEPENDENCIES?

[itchyny]

Hmm, I configure as CONFIG_STATUS_DEPENDENCIES = $(SOURCES), but PACKAGE_VERSION is cached in autom4te.cache/ and the version is not re-evaluated. The package version in AC_INIT is fairly static, and we can't easily allow make to update the version when the git revision (or dirty state) has changed. I understand why we had special rules to make version.h, and regret not confirming well before merging this PR, but let's wait for the author's comment.