build debug version failed

[alongL]

If we build debug version.

mkdir build
cmake -DCMAKE_BUILD_TYPE=Debug ..
make -j

There will be an error.

/usr/bin/ld: ../source/libtengine-lite.so: undefined reference to `enable_mem_stat'
/usr/bin/ld: ../source/libtengine-lite.so: undefined reference to `disable_mem_stat'

in source/utility/mem_stat.c enable_mem_stat is set to extern. Is it defined in another file?

#ifdef CONFIG_MEM_STAT

extern void (*enable_mem_stat)(void);
extern void (*disable_mem_stat)(void);

Or should we define it like this here ? (delete extern)

#ifdef CONFIG_MEM_STAT

void (*enable_mem_stat)(void);
void (*disable_mem_stat)(void);