configure: Check for -Werror parameters on clang

Signed-off-by: Martin Storsjö <martin@martin.st>

configure

@@ -3549,6 +3549,8 @@ elif enabled llvm_gcc; then
 elif enabled clang; then
     check_cflags -mllvm -stack-alignment=16
     check_cflags -Qunused-arguments
+    check_cflags -Werror=implicit-function-declaration
+    check_cflags -Werror=missing-prototypes
 elif enabled armcc; then
     # 2523: use of inline assembler is deprecated
     add_cflags -W${armcc_opt},--diag_suppress=2523