cmake: propagate public header directory to target consumers

[fhgffy]

A parent project using add_subdirectory(libqrencode) and target_link_libraries(app qrencode) cannot include <qrencode.h> without adding libqrencode's source directory manually. The library builds, but the consumer fails with fatal error: qrencode.h: No such file or directory. This affects both static and shared builds.

Attach the public header directory to qrencode as a build-tree interface include directory. Add a CMake consumer regression that includes the public header, links only the library target, encodes a string, and frees the result. Include its source in Automake's EXTRA_DIST so source distributions retain the CMake test.

The existing tests mostly include the header using a relative path and therefore do not catch this missing target usage requirement. Subdirectory use was previously supported by #133.

Validation on Linux with GCC 14.2.0, CMake 3.11.2 and 3.31.6:

• Static and shared parent consumers fail before the fix and build/run afterward; the new regression also fails when the production change is removed
• Direct and transitive target consumers pass, including paths containing spaces
• Original CTest suite: 12/12 pass; fixed static Debug and shared Release suites: 13/13 pass each
• Installed-header/library consumers and the PNG-disabled CLI still build and run

The patch received an independent review and a fresh replay of all 48 validation commands with CMake 3.31.6. Windows, macOS, CMake 3.1, PNG-enabled tools, and Autotools make dist were not run locally. CMake 4's existing minimum-version configuration failure affects both base and fixed code and remains separate in #235.

Prepared with AI assistance.