Work around -Werror problems when detecting qsort variants

If `ENABLE_WERROR` is on, the CMake configure tests for the `qsort_r`
and `qsort_s` variants may fail due to warnings about unused functions
or unused parameters. These warnings can be ignored, so disable them
specifically for running those tests.

src/CMakeLists.txt

@@ -58,6 +58,13 @@ add_feature_info(futimens GIT_USE_FUTIMENS "futimens support")
 
 # qsort
 
+# for these tests, temporarily save CMAKE_C_FLAGS and disable warnings about
+# unused functions and parameters, otherwise they will always fail if
+# ENABLE_WERROR is on
+set(SAVED_CMAKE_C_FLAGS "${CMAKE_C_FLAGS}")
+disable_warnings(unused-function)
+disable_warnings(unused-parameter)
+
 # old-style FreeBSD qsort_r() has the 'context' parameter as the first argument
 # of the comparison function:
 check_prototype_definition(qsort_r
@@ -82,6 +89,9 @@ check_prototype_definition(qsort_s
 	"void (qsort_s)(void *base, size_t num, size_t width, int (*compare )(void *, const void *, const void *), void *context)"
 	"" "stdlib.h" GIT_QSORT_S_MSC)
 
+# restore CMAKE_C_FLAGS
+set(CMAKE_C_FLAGS "${SAVED_CMAKE_C_FLAGS}")
+
 # random / entropy data
 
 check_function_exists(getentropy GIT_RAND_GETENTROPY)