# 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

Triggered-By:	cmake	d5558e3ad3c2cdabfb1ba31d20ea4defce570a95
Upstream-PR:	http://github.com/swift-nav/cmake/pull/188