cmake: fix curl-config --static-libs

[Lekensteyn]

Libraries for curl-config --static-libs are required to have the form
-ldl. Assume library paths like /usr/lib/libssl.so and map it to "ssl"
for curl-config.

This removes the confusing messag "Static linking is broken" which was
printed because curl-config --static-libs was disfunctional while the
static libcurl.a is fine.

Fixes #841

[mention-bot]

By analyzing the blame information on this pull request, we identified @billhoffman, @jzakrzewski and @Sukender to be potential reviewers

[bagder]

I'd like a second cmake fluent person to +1 this PR before I consider merging it. @snikulov perhaps?

[snikulov]

@bagder Will check it soon.

[snikulov]

@bradking Could you also take a look on this PR?

[snikulov]

Not really useful for MS platform, because of sh script.
Changes targeted to curl-config generation which is, I think, should be reworked per #885
If anyone use it, they should check this change also.

For me only one line is fine, where the message is removed.
👍 for that.

[bradking]

Does curl-config --static-libs have to print -lfoo instead of /path/to/libfoo.so? Clients should link just fine with the absolute paths. IIUC the --static-libs option is about linking to libcurl.a plus whatever dependencies it has, whether or not they are shared or static libs. One could construct LIBCURL_LIBS by checking for / in library names to decide whether to use -l as a prefix.

[Lekensteyn]

@bradking Oh, passing /usr/lib/libfoo.so rather than -lfoo seems to work (well, checked it with gcc -v test.c -lcurl vs. the absolute path). Is this a universal thing? It sounds like a good idea to prepend -l only if there is no / in the name.

[bradking]

@Lekensteyn yes, passing the absolute path is universally supported. CMake tries to do this whenever possible because it ensures that the desired library file is used instead of just hoping that -lfoo finds the right one.

[Lekensteyn]

Sorry for the delay, I have updated the patch according to @bradking's suggestion and now curl-config shows:

/usr/local/lib/libcurl.a -lc -lidn -llber -lldap -ldl /usr/lib64/libssl.so /usr/lib64/libcrypto.so /usr/lib64/libz.so /usr/lib64/libssh2.so

This should work better.

[bradking]

LGTM.

[Lekensteyn]

@snikulov Is this patch OK to merge? You referenced #885, but that is about a new cmake file for use with find_package. This change just provided compatibility for the curl-config tool on *nix.

[snikulov]

@Lekensteyn Yes. LGTM.

[Lekensteyn]

Applied in 2f3feda