configure step and "implicit-function-declaration"

This breaks :

./configure CFLAGS="-O2 -pipe -march=native -Werror=implicit-function-declaration"
...
checking for pthread_create... yes
checking for pthread_condattr_setclock... yes
checking for libevent directory... configure: WARNING: We found the libraries for libevent, but we could not find the C header files.  You may need to install a devel package.
configure: error: Missing headers; unable to proceed.

but neither this :

./configure --enable-gcc-warnings

nor this

./configure CFLAGS="-O2 -pipe -march=native" --enable-gcc-warnings

Shouldn't the later break too ?

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