cmake-env.mk: Allow using BUILD_SHARED_LIBS and fix legacy behavior

[th0ma7]

Description

cmake-env.mk: Allow using BUILD_SHARED_LIBS and fix legacy behavior to not override shared library definition.

Relates to #6415

Checklist

• Build rule all-supported completed successfully
• New installation of package completed successfully
• Package upgrade completed successfully (Manually install the package again)
• Package functionality was tested
• Any needed documentation is updated/created

Type of change

• Bug fix
• New Package
• Package update
• Includes small framework changes
• This change requires a documentation update (e.g. Wiki)

[hgy59]

@th0ma7 This solution is only half the truth.

Good, you fixed the use of BUILD_SHARED_LIBS for builds without toolchain files.

But what, if you must suppress the definition of -DBUILD_SHARED_LIBS at all?

The definition of -DBUILD_SHARED_LIBS=OFF for transmission is only a workaround.
The real solution would be to omit the definition.

IMHO the -DBUILD_SHARED_LIBS should be used when building libraries only.
transmission is not a library but an executable that contains several third-party libraries in its own source code.

So I miss an option like BUILD_SHARED_LIBS = DISABLED to avoid adding -DBUILD_SHARED_LIBS to CMAKE_ARGS.

In the transmission package this popped up, because of an upstream issue.
For other third-party libraries transmission has dedicated options to create static libs (like -DUPNPC_BUILD_STATIC=ON or -DLIBDEFLATE_BUILD_SHARED_LIB=OFF), but this is missing for the crc32c lib, that was added recently.
transmission has also third-party libraries that are used as shared libraries and it's pure chance that our workaround works.

[th0ma7]

When I first added CMake functionality years ago I somehow enforced BUILD_SHARED_LIBS to be always ON unless already set (no clue why, probably thought it made sense). Later I added meson and I did not enforce building as shared library by default (neither was the original framework using GNU configure).

Issue may be twofold:

1. It is feasible to add a workaround to disable this flag altogether, but that would really just be a workaround. I am rather presuming transmission has an issue within its CMake configuration files.
2. We could remove that shared libs option from the default CMake configuration altogether, assuming a similar behavior than GNU configure and meson. Although this "might" break many packages (or not) where the fix would be to re-add it to their respective makefile...

@hgy59 Also, to return to your point above, I don't believe there is any way to set shared library building only for libraries per-say. For transmission specifically, couldn't we simply patch the code to add a -DCRC32C_BUILD_STATIC=ON or similar?