Hi,
FYI:
I just had a run-in with a cmake-based project that found the correct libpng
binary during the configure phase but for a rather nasty reason tried to link
to another version installed elsewhere. That was on Linux, but there's no
guarantee that the same issue cannot occur on OS X.
- find(PNG) correctly set PNG_LIBRARIES to
`$prefix/lib/libpng.dylib:$prefix/lib/libz.dylib`
- however, the compiler/linker command had `-lpng -lz` instead, and no
preceding -L$prefix/lib and thus found libpng in a standard location (I learned
that CMake calls those "implicit link directories").
Normally this would have been fine because "base" sets LIBRARY_PATH to
$prefix/lib, and indeed that suffices with gcc. Not so with clang (tested only
with 4.0).
When diagnosing this I noticed that
- gcc translates LIBRARY_PATH=/path to -L/path BEFORE the first -lfoo option on
the actual linker commandline
- clang does something similar, but adds the -L/path AFTER the -lfoo options
given by the user.
IOW, with clang, LIBRARY_PATH only affects the search path for libraries added
automatically, not those specified by the user.
To cut a long story short, it is possible to deactivate this automatic libspec
rewriting (see the `CMAKE_<LANG>_IMPLICIT_LINK_DIRECTORIES` documentation) by
setting policy CMP0060 to NEW.
I've done this in the still-pending update to my cmake-1.1 PG by adding
`-DCMAKE_POLICY_DEFAULT_CMP0060=NEW` to the default configure.args list.
In the case of the project in question (and linking to libpng in general), one
also avoids the issue by using `PNG::PNG` instead of `${PNG_LIBRARIES}`.
R.
