Fix compiler errors from check third-party dependency on Windows (#188)

# Issue

The check unit testing framework (used as a third-party dependency)
emits compiler diagnostics that break the build on Windows platforms,
even though it's configured as an external dependency with
`SYSTEM_INCLUDES`.

Specific error:
``` 
D:\a\libsbp\libsbp\c\third_party\check\lib\timer_delete.c:48:5: error: implicit declaration of function 'alarm' [-Wimplicit-function-declaration]
```

# Root Cause

The check library has platform compatibility issues on Windows:

1. Missing POSIX functions: Windows lacks POSIX functions like alarm(),
setitimer(), and proper timer support
2. Incomplete fallback detection: While check includes compatibility
shims (e.g., alarm.c), the file timer_delete.c calls alarm() directly
without proper header inclusion or declaration checking
3. SYSTEM_INCLUDES limitation: The SYSTEM_INCLUDES flag in CMake only
suppresses warnings from headers consumed via
target_include_directories(). However, check adds its compatibility
source files (including timer_delete.c) directly to the target via
target_sources(), so these files are compiled as part of the target
itself and inherit all parent compile flags including -Werror

Note: The check project has been unmaintained for over 4 years (last
release: 0.15.2 in 2020), making an upstream fix unlikely.

# Solution

Modified cmake/common/FindCheck.cmake to temporarily adjust
CMAKE_C_FLAGS during the check library's configuration and compilation:

1. Save original flags before including the dependency
2. Apply platform-specific suppressions:
- MinGW/GCC: Remove -Werror and add
-Wno-error=implicit-function-declaration
- MSVC: Remove /WX and add /wd4013 (disables C4013: implicit function
declaration)
3. Restore original flags immediately after the dependency is added

This approach:
 - ✅ Doesn't modify third-party code
 - ✅ Only affects check library compilation
 - ✅ Preserves strict warnings for project code
 - ✅ Supports both MinGW and MSVC toolchains

Author: sbmueller

FindCheck.cmake

@@ -13,7 +13,29 @@
 include("GenericFindDependency")
 option(CHECK_ENABLE_TESTS "" OFF)
 option(CHECK_INSTALL "" OFF)
+
+# Store current compile options to restore later
+set(_SAVED_CMAKE_C_FLAGS "${CMAKE_C_FLAGS}")
+
+# Temporarily modify CMAKE_C_FLAGS to disable implicit-function-declaration errors
+# for the check library compilation (needed for Windows compatibility)
+if(WIN32)
+  if(MSVC)
+    # MSVC: Remove warnings-as-errors flag and disable specific warning C4013 (implicit function declaration)
+    string(REPLACE "/WX" "" CMAKE_C_FLAGS "${CMAKE_C_FLAGS}")
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} /wd4013")
+  else()
+    # MinGW/GCC: Remove -Werror and disable implicit-function-declaration errors
+    string(REPLACE "-Werror" "" CMAKE_C_FLAGS "${CMAKE_C_FLAGS}")
+    set(CMAKE_C_FLAGS "${CMAKE_C_FLAGS} -Wno-error=implicit-function-declaration")
+  endif()
+endif()
+
 GenericFindDependency(
   TARGET check
   SYSTEM_INCLUDES
 )
+
+# Restore original compile flags
+set(CMAKE_C_FLAGS "${_SAVED_CMAKE_C_FLAGS}")
+unset(_SAVED_CMAKE_C_FLAGS)