cmake: Set top-level target output locations

This change:
1. Collects build artifacts in dedicated locations.
2. Allows to run individual examples with a shared library on Windows.
3. Is compatible with Wine when testing cross-compiled Windows binaries
   on Linux.
4. Is compatible with integration the project into a larger project
   hierarchy.

.github/workflows/ci.yml

@@ -751,14 +751,14 @@ jobs:
         # Use the bash shell included with Git for Windows.
         shell: bash
         run: |
-          cd build/src/RelWithDebInfo && file *tests.exe bench*.exe libsecp256k1-*.dll || true
+          cd build/bin/RelWithDebInfo && file *tests.exe bench*.exe libsecp256k1-*.dll || true
 
       - name: Check
         run: |
           ctest -C RelWithDebInfo --test-dir build -j ([int]$env:NUMBER_OF_PROCESSORS + 1)
-          build\src\RelWithDebInfo\bench_ecmult.exe
-          build\src\RelWithDebInfo\bench_internal.exe
-          build\src\RelWithDebInfo\bench.exe
+          build\bin\RelWithDebInfo\bench_ecmult.exe
+          build\bin\RelWithDebInfo\bench_internal.exe
+          build\bin\RelWithDebInfo\bench.exe
 
   win64-native-headers:
     name: "x64 (MSVC): C++ (public headers)"

CMakeLists.txt

@@ -286,6 +286,15 @@ if(SECP256K1_APPEND_LDFLAGS)
   string(APPEND CMAKE_C_LINK_EXECUTABLE " ${SECP256K1_APPEND_LDFLAGS}")
 endif()
 
+if(NOT CMAKE_RUNTIME_OUTPUT_DIRECTORY)
+  set(CMAKE_RUNTIME_OUTPUT_DIRECTORY ${PROJECT_BINARY_DIR}/bin)
+endif()
+if(NOT CMAKE_LIBRARY_OUTPUT_DIRECTORY)
+  set(CMAKE_LIBRARY_OUTPUT_DIRECTORY ${PROJECT_BINARY_DIR}/lib)
+endif()
+if(NOT CMAKE_ARCHIVE_OUTPUT_DIRECTORY)
+  set(CMAKE_ARCHIVE_OUTPUT_DIRECTORY ${PROJECT_BINARY_DIR}/lib)
+endif()
 add_subdirectory(src)
 if(SECP256K1_BUILD_EXAMPLES)
   add_subdirectory(examples)