M: fix test source file to be compiled even with -Werror C flag

modules/find/FindM.cmake

@@ -38,8 +38,7 @@ include(FindPackageHandleStandardArgs)
 
 # check that we can call math directly with the compiler
 include(CheckCSourceCompiles)
-set(LIBM_TEST_SOURCE "#include<math.h>\nfloat f; int main(){sqrt(f);return
-0;}")
+set(LIBM_TEST_SOURCE "#include<math.h>\nfloat f; int main(){float s=sqrt(f);return 0;}")
 check_c_source_compiles("${LIBM_TEST_SOURCE}" HAVE_MATH)
 
 # if works with the compiler we do not need anything else, variables are empty
@@ -81,4 +80,4 @@ else()
     endif()
   endif()
 
-endif()
\ No newline at end of file
+endif()