libssh2.pc: re-add & extend support for static-only libssh2 builds

[vszakats]

Adapted for libssh2 from the curl commit message by James Le Cuirot:

"A project built entirely statically will call pkg-config with
--static, which utilises the Libs.private: field. Conversely it will
not use --static when not being built entirely statically, even if
there is only a static build of libssh2 available. This will most
likely cause the build to fail due to underlinking unless we merge the
Libs: fields.

Consider that this is what the Meson build system does when it generates
pkg-config files."

This patch extends the above to Requires:, to mirror Libs: with
pkg-config package names.

Follow-up to 1209c16 #1114

Ref: #1114 (comment)
Ref: curl/curl@98e5904
Ref: curl/curl#5373
Closes #1119

[vszakats]

Misc observations comparing libssh2.pc generated by autotools and CMake and with libcurl.pc:

• libssh2.pc does have a Requires.private on top of Libs.private (compared to curl, where Requires.private is missing, and adding it is a KNOWN_BUGS item: consider utilizing Requires.private directives in libcurl.pc instead of Libs.private curl/curl#864 https://curl.haxx.se/docs/knownbugs.html#Utilize_Requires_private_directi

• Requires.private refers to libssl,libcrypto instead of e.g. openssl (with both autotools and CMake). I may be misunderstaning, but this line is supposed to refer to valid pkg-config package names, instead of libnames. ...Well, specifically for libssl and libcrypto, OpenSSL provides such .pc files on top of openssl.pc, so this case is fine. No other crypto backens are added there by CMake though, and only libwolfssl is added by autotools (the correct name seems to be wolfssl as in wolfssl.pc, the file provided by wolfSSL) [build: add/fix Requires.private packages in libssh2.pc #1123].

• Requires.private uses space separators with autotools, commas with CMake. Commas are the official separator, but both are reported to be parsed correctly by pkg-config. [autotools: use comma separator in Requires.private of libssh2.pc #1124]

• Libs.private mirrors Requires.private with autotools, but it's left empty by CMake.

• Libs.private is populated with Windows system libs by both Cmake and autotools. autotools correctly picks up -lucrt, CMake doesn't.

• Requires is not used. Though it seems the reason we mirror Libs.private to Libs (in this PR) can also apply to Requires*. [libssh2.pc: re-add & extend support for static-only libssh2 builds #1119] Ref: Question: correct way to use Requires.private pkgconf/pkgconf#300 (comment)

• @libdir@ is replaced with exec_prefix by autotools and prefix by CMake (in the specific case I checked).

• Libs.private is populated by autotools with OpenSSL libs, CMake doesn't add them. [cmake: add openssl libs to Libs.private #1127]

• Libs.private is populated by autotools with detected crypto libs, CMake ignores detection results and uses hard-coded values instead.

Ref:

• https://people.freedesktop.org/~dbn/pkg-config-guide.html
• https://www.freedesktop.org/wiki/Software/pkg-config/
• https://linux.die.net/man/1/pkg-config