configure: build with -O2 and -pipe if supported.

configure

@@ -264,6 +264,8 @@ if [ $? -eq 0 ]; then
 fi
 check_compiler_flag "std=c99" "" CFLAGS
 check_compiler_flag "l,--as-needed" W LDFLAGS
+check_compiler_flag "O2" "" CFLAGS
+check_compiler_flag "pipe" "" CFLAGS
 
 #
 # Check if -fPIE and -pie are supported if --build-pie is set.