cmake: Fix toolchain.cmake for *BSD systems

[hebasto]

For *BSD systems, the qt and qrencode packages are not built, even without the NO_QT or NO_QR variables defined, because these package are not available for the host systems:

bitcoin/depends/packages/packages.mk

Lines 7 to 13 in 19d4d92

	qrencode_linux_packages = qrencode
	qrencode_darwin_packages = qrencode
	qrencode_mingw32_packages = qrencode
	qt_linux_packages:=qt expat libxcb xcb_proto libXau xproto freetype fontconfig libxkbcommon libxcb_util libxcb_util_render libxcb_util_keysyms libxcb_util_image libxcb_util_wm
	qt_darwin_packages=qt
	qt_mingw32_packages=qt

We already handle the exact logic for the systemtap (USDT) package. Therefore, it is reasonable to use this working logic uniformly for all cases.

Additionally, this PR correctly handles cases when a *_packages_ variable contains only spaces rather than being empty.

---

How to test this PR on a *BSD system:

1. Build depends following the build guides (depends/README.md, doc/build-*bsd.md)
2. Configure the main build system using the generated toolchain file:

cmake -B build --toolchain depends/<your-platform>/toolchain.cmake

It fails.

With this PR it configures successfully.

[hebasto]

Friendly ping @theStack @vasild :)

[vasild]

ACK 20ef783

... because these package are not available for the host systems ...

Why? FreeBSD has packages for qt and qrencode.

[hebasto]

... because these package are not available for the host systems ...

Why? FreeBSD has packages for qt and qrencode.

I did not mean the package availabilty via the system package manager, but rather how our depends build sub-system is configured.

[hebasto]

@vasild

Testing instructions have been added to the PR description per your request.

[vasild]

I tested this on FreeBSD: on cmake-staging it fails with:

-- Checking for module 'libqrencode'
--   Package 'libqrencode' not found
CMake Error at /usr/local/share/cmake/Modules/FindPkgConfig.cmake:634 (message):
  The following required packages were not found:

   - libqrencode

and succeeds with the changes from this branch.

Unrelated to this PR: depends/README.md mentions to run make followed by cmake -B build ... but those have to be run from different directories - make has to be in the depends/ subdirectory. On FreeBSD make spits a ton of syntax errors because depends/Makefile is gnu-make specific, so I used gmake -C depends from the top source directory.

[theStack]

post-merge ACK 20ef783, tested on OpenBSD 7.5
(note that I had to cherry-pick the zeromq build fix for OpenBSD [bitcoin#30565, 89b1d5c] first, as it's seemingly not in the cmake staging branch yet)