Enhance build experience without vcpkg

[Fat-Zer]

• make some options default to ON only when building with vcpkg
• avoid using find_package() for liblcms2 as most of distributions don't provide it, especially the config version.
• use CMAKEINSTALL* for default dir variables instead of hardcoding in lib/bin/share. This will automatically set correct libdir on most 64-bit systems (lib64) and also will allow user to override those from the command line.
• avoid resetting default CMAKE_INSTALL_PREFIX to a path inside the builddir, as it usually expected. It's supposed to be directory where program will reside when it is installed on an end-user system. To preliminary install it to a different directory you supposed to use cmake --install . --prefix /temporary/install/path

---

I believe all this changes are generally beneficial, but I'm open to discussion.

PS: I haven't tested whether it doesn't actually break build WITH vcpkg, but I believe it shouldn't. PPS: VCPKG_TOOLCHAIN is supposed to be define by $VCPKG_ROOT/scripts/buildsystems/vcpkg.cmake

[Fat-Zer]

missed a line in the commit, hence the force-pushed

[JakubMelka]

@Fat-Zer , could you please sign a contributor license agreement?

[Fat-Zer]

@JakubMelka, probably, how exactly I can do it?

[JakubMelka]

Just download https://github.com/JakubMelka/PDF4QT/blob/master/CLA/Contributor_License_Agreement.pdf, sign it and send me the signed PDF via email. Thank you!

[Fat-Zer]

Just download https://github.com/JakubMelka/PDF4QT/blob/master/CLA/Contributor_License_Agreement.pdf, sign it and send me the signed PDF via email. Thank you!

Sorry, I'm ok with signing-off my contribution and granting you right to sublicence it however you want, but I don't want to reveal my address, my date of birth or my signature.

This whole process looks like as an over-complicated legalistically-bureaucratic hassle and just somewhat bizarre. I was expecting some more common form of waving away my rights like adding a tick to the PR that I'm agree with terms, signing-off the commits or at most some online form.

Also speaking purely hypothetical, I've got some issues with wording of 3.3: it sounds like if a third party will make some claims about my contribution I will be liable for your damages even if the claims are absolutely false.