Fix shared build

[intelligide]

Currently, it always build both static and shared libs with the same name, which leads to name collision on Windows. The static library cbor.a replaces the symbol archive cbor.a.

This PR fix this problem:

• Build shared lib or static lib, not both at once. Use BUILD_SHARED_LIBS to create the shared library.
• Export symbols with cbor_export.h header (generated by CMake) and CBOR_EXPORT macros.

Fix conan-io/conan-center-index#2578

[codecov-commenter]

Codecov Report

Merging #148 into master will decrease coverage by 1.72%.
The diff coverage is 91.30%.

@@            Coverage Diff             @@
##           master     #148      +/-   ##
==========================================
- Coverage   90.16%   88.44%   -1.73%     
==========================================
  Files          52       53       +1     
  Lines        3528     3548      +20     
  Branches      195      198       +3     
==========================================
- Hits         3181     3138      -43     
- Misses        169      332     +163     
+ Partials      178       78     -100     

Impacted Files	Coverage Δ
src/allocators.c	100.00% <ø> (ø)
src/cbor/callbacks.c	100.00% <ø> (ø)
test/memory_allocation_test.c	83.00% <ø> (ø)
src/cbor/internal/builder_callbacks.c	90.08% <86.66%> (+3.68%)	⬆️
src/cbor/internal/stack.c	93.75% <100.00%> (+0.41%)	⬆️
test/stack_over_limit_test.c	100.00% <100.00%> (ø)
test/assertions.c	0.00% <0.00%> (-100.00%)	⬇️
test/stream_expectations.c	0.00% <0.00%> (-100.00%)	⬇️
src/cbor/strings.c	96.00% <0.00%> (+1.33%)	⬆️
... and 12 more

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[PJK]

Hi @intelligide, thank you very much for the PR, I think this makes sense.

Quick question: What is the potential to cause breakages/incompatibilities for clients that rely on the current behavior? I.e. do we need a new version?

[intelligide]

There is no breaking change inside code. And if clients currently use the static lib, all should be fine.
A minor version be good.

[PJK]

Thank you, I'll push a release next week.

One more question: Is it possible to write a "linkage test" that would prevent future regressions (e.g. someone forgets to export a new function)?

[intelligide]

Just write tests for each function and run them twice with BUILD_SHARED_LIBS as ON and OFF. The build will fail if an used function isn't exported.