opendmarc -V: show configure args, git hash, and clarify SPF backend

[thegushi]

Summary

Three related improvements to -V output, mirroring what was done in OpenDKIM (#366):

• CONFIGURE_ARGS: ./configure arguments are now embedded via AC_DEFINE_UNQUOTED, so operators can see exactly how the binary was built.
• Git hash in version string: configure.ac runs git describe --tags --dirty; if the result differs from PACKAGE_VERSION (i.e. the build is not an exact tagged release), it becomes DMARCF_VERSION. Tagged release builds and environments without git fall back to the plain version number. All five VERSION references in opendmarc.c replaced with DMARCF_VERSION.
• SPF backend clarity: the redundant WITH_SPF + WITH_SPF2 optlist entries are merged into a single WITH_SPF (libspf2) or WITH_SPF (internal) string.

Example output (dev build with SPF + libspf2)

opendmarc: OpenDMARC Filter v1.4.2-5-gabcdef7
    SMFI_VERSION 0x1000002
    libmilter version 1.0.2
    Configured with:  '--with-spf' '--with-spf2-include=/usr/local/include/spf2' '--with-spf2-lib=/usr/local/lib'
    Active code options:
        WITH_SPF (libspf2)

Test plan

• Build from a tagged commit: version string is plain 1.4.2
• Build from an untagged commit: version string is 1.4.2-N-gHASH (or -dirty suffix if tree is modified)
• Build without git in PATH: falls back to plain version number
• Build without SPF: no SPF entry in active options, Configured with: reflects flags used
• Build with --with-spf: shows WITH_SPF (internal)
• Build with --with-spf --with-spf2-include=... --with-spf2-lib=...: shows WITH_SPF (libspf2)
• CI passes