autotools: generated libssh2.pc file broken

[haampie]

Describe the bug
When using the autotools build system with libssh 1.11, and OpenSSL 3

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the generated pc file looks like this:

###########################################################################
# libssh2 installation details
###########################################################################

prefix=/home/harmen/spack/opt/spack/linux-ubuntu22.10-zen2/gcc-12.2.0/libssh2-1.11.0-zg3jzgpwy3ectyavqw6yrtfujchcyl6x
exec_prefix=${prefix}
libdir=${exec_prefix}/lib
includedir=${prefix}/include

Name: libssh2
URL: https://www.libssh2.org/
Description: Library for SSH-based communication
Version: 1.11.0
Requires.private: libssl libcrypto zlib
Libs: -L${libdir} -lssh2  -lssl -lcrypto
Libs.private:  -lssl -lcrypto
Cflags: -I${includedir}

It includes -lssl and -lcrypto, which is wrong in two ways:

1. The -L flag is missing if OpenSSL is in an non-default directory;
2. The libraries shouldn't be there at all? Given that they're part of OpenSSL, not libssh2. [Fixed in Don't put @LIBS@ in pc file #1114]

Compare this to the CMake generated pc file:

###########################################################################
# libssh2 installation details
###########################################################################

prefix=/home/harmen/spack/opt/spack/linux-ubuntu22.10-zen2/gcc-12.2.0/libssh2-1.11.0-nfwxj7fefm65hmotelkban26t5bf3qcm
exec_prefix=${prefix}
libdir=${prefix}/lib
includedir=${prefix}/include

Name: libssh2
URL: https://www.libssh2.org/
Description: Library for SSH-based communication
Version: 1.11.0
Requires.private: libssl,libcrypto,zlib
Libs: -L${libdir} -lssh2 
Libs.private: 
Cflags: -I${includedir}

Expected behavior
I would expect the Autotools-generated pc file to be the same as the CMake one.

Additionally, it seems that the pc file contents depend on whether you install openssl with --libdir=lib64 vs --libdir=lib.

• Using --libdir=lib64 (default in OpenSSL 3?) there are no -L flags added to the pc file when using autotools
• Using --libdir=lib the are -L flags added to the pc file when using autotools.

[vszakats]

Checked curl; there is an elaborate custom script there to detect OpenSSL: https://github.com/curl/curl/blob/3b3c41f9337779723d0236c74c3fba14776dc702/m4/curl-openssl.m4#L28
(most likely overkill for libssh2 as-is)

Also for libssh2 we only need -lcrypto, never -lssl. [some of this fixed in #1128]