build: miscellaneous fixes

[thesamesam]

Thanks for the new release!

Upstreaming a few patches we've been carrying downstream in Gentoo.

• Rename configure.in -> configure.ac to non-deprecated filename
• Respect CPPFLAGS, LDFLAGS
• Respect AR

[thesamesam]

ping @Mraoul

[malwarefrank]

Thank you for the PR and sorry it has been a while. I plan to review this soon.

[thesamesam]

Thank you!