libframe: fix bugs in cmake configuration

[duncanmmacleod]

Description

This PR fixes problems with the cmake configuration for libframe introduced by #27384.

Type(s)

• bugfix
• enhancement
• security fix

Tested on

macOS 12.7.6 21H1320 arm64
Xcode 14.1 14B47b

Verification

Have you

• followed our Commit Message Guidelines?
• squashed and minimized your commits?
• checked that there aren't other open pull requests for the same change?
• referenced existing tickets on Trac with full URL in commit message?
• checked your Portfile with port lint?
• tried existing tests with sudo port test?
• tried a full install with sudo port -vst install?
• tested basic functionality of all binary files?
• checked that the Portfile's most important variants haven't been broken?

[macportsbot]

Notifying maintainers:
@lpsinger for port libframe.