fix --enable-debug option

[f37d8506e0cd769acd026517074ab79a86cfd8a4] Make CFLAGS overridable
borked --enable-debug[=yes] by causing builds to always compile with ` std=gnu99 -g3 -O0 -pthread -g -O2 -g -Wall`.

Debug symbols are indeed included but the binary is always built optimized.