Merge bitcoin-core/secp256k1#1367: build: Improvements to symbol visibility logic on Windows (attempt 3)

c6cd2b15a0 ci: Add task for static library on Windows + CMake (Hennadii Stepanov)
020bf69a44 build: Add extensive docs on visibility issues (Tim Ruffing)
0196e8ade1 build: Introduce `SECP256k1_DLL_EXPORT` macro (Hennadii Stepanov)
9f1b1904a3 refactor: Replace `SECP256K1_API_VAR` with `SECP256K1_API` (Hennadii Stepanov)
ae9db95cea build: Introduce `SECP256K1_STATIC` macro for Windows users (Hennadii Stepanov)

Pull request description:

  Previous attempts:
  - https://github.com/bitcoin-core/secp256k1/pull/1346
  - https://github.com/bitcoin-core/secp256k1/pull/1362

  The result is as follows:
  1. Simple, concise and extensively documented code.
  2. Explicitly documented use cases with no ambiguities.
  3. No workarounds for linker warnings.
  4. Solves one item in https://github.com/bitcoin-core/secp256k1/issues/1235.

ACKs for top commit:
  real-or-random:
    utACK c6cd2b15a0

Tree-SHA512: d58694452d630aefbd047916033249891bc726b7475433aaaa7c3ea2a07ded8f185a598385b67c2ee3440ec5904ff9d9452c97b0961d84dcb2eb2cf46caa171e

.cirrus.yml

@@ -384,12 +384,17 @@ task:
     # Ignore MSBuild warning MSB8029.
     # See: https://learn.microsoft.com/en-us/visualstudio/msbuild/errors/msb8029?view=vs-2022
     IgnoreWarnIntDirInTempDetected: 'true'
+  matrix:
+    - env:
+        BUILD_SHARED_LIBS: ON
+    - env:
+        BUILD_SHARED_LIBS: OFF
   git_show_script:
     # Print commit to allow reproducing the job outside of CI.
     - git show --no-patch
   configure_script:
     - '%x64_NATIVE_TOOLS%'
-    - cmake -E env CFLAGS="/WX" cmake -G "Visual Studio 17 2022" -A x64 -S . -B build -DSECP256K1_ENABLE_MODULE_RECOVERY=ON -DSECP256K1_BUILD_EXAMPLES=ON
+    - cmake -E env CFLAGS="/WX" cmake -A x64 -B build -DSECP256K1_ENABLE_MODULE_RECOVERY=ON -DSECP256K1_BUILD_EXAMPLES=ON -DBUILD_SHARED_LIBS=%BUILD_SHARED_LIBS%
   build_script:
     - '%x64_NATIVE_TOOLS%'
     - cmake --build build --config RelWithDebInfo -- -property:UseMultiToolTask=true;CL_MPcount=5