Some polishing for the cmake ReSVG discovery

[ferdnyc]

find_package_handle_standard_args is supposed to set the _FOUND variable automatically (as the comment right above it says), as well as handling things like REQUIRED, QUIETLY, etc. It should always be run at the end of a module, for this reason.

This change removes the conditionals around the call, lets it handle what it's meant to handle, and defines a custom failure message for discovery that replaces the manual one in src/CMakeList.txt.

In addition, the REQUIRED is removed from tests/CMakeLists.txt, since it's supposed to mark the module as required (which ReSVG is not), and was only working due to the aforementioned improper
conditional wrapping of the module's cleanup.

With these changes, on my system libopenshot still builds and make test still runs successfully both with and without ReSVG being discovered.

Additionally, if only one of the two parts is missing, cmake will automatically indicate that in the failure message:

$ cd /tmp/resvg/target/release/
$ ln -s ../../capi/include .
$ ls
build  examples  incremental  libresvg.d        libresvg.rlib  libusvg.rlib
deps   include   libresvg.a   libresvg_qt.rlib  libresvg.so    native
$ rm libresvg.so libresvg.a # (To my great surprise, libresvg.a would be discovered and used!)
$ cd /tmp/libopenshot/
$ rm CMakeCache.txt
$ RESVGDIR=/tmp/resvg/target/release /usr/bin/cmake -DCMAKE_INSTALL_PREFIX:PATH=/usr
[...]
-- Could NOT find RESVG, using Qt SVG parsing instead (missing: RESVG_LIBRARY) 

The message correctly indicates that only RESVG_LIBRARY was not found, because RESVG_INCLUDE_DIR was still successfully discovered as /tmp/resvg/target/release/include/.

Also:

• $ENV{RESVGDIR} is added to the search list for find_path( RESVG_LIBRARY..., so that copying/linking the include dir into target/release/ and running RESVGDIR=/path/to/resvg/target/release/ cmake ... will successfully discover the library
• The comment in FindRESVG.cmake claiming it sets RESVG_DEFINITIONS is removed, because it doesn't

[jonoomph]

This looks great @ferdnyc! I'm gonna test this on my system real quick

[jonoomph]

Worked great on my system! Merged and hopefully all the build servers will like it also. 😄