Fix compilation with -Wunused-variable

When compiling with -Wunused-variable, `EXT_STRERROR_R_CHAR_P` gets undefined because of a failing compilation check.

An alternative would be to mark `c` as `__attribute__((undefined))`, but MSVC doesn't have `__attribute__((undefined))`.

cmake/generateConfigFile.cmake

@@ -32,6 +32,7 @@ check_cxx_source_compiles( "
 int main() {
     char buff[100];
     const char* c = strerror_r(0,buff,100);
+    (void)c;  // ignore unuse-variable
     return 0;
 }" EXV_STRERROR_R_CHAR_P )