### Description

Currently, an undefined environmental variable with `#!defenv` results in Kamailio failing to start. The proposal here that if the environmental variable is not defined, then the macro value would be undefined as well. For example:

``` !#KAMAILIO loadmodule "xlog" loadmodule "pv"

#!defenv FOO

request_route { #!ifdef FOO xlog("L_INFO","Environmental variable $$FOO is set\n"); #!else xlog("L_WARN","Environmental variable $$FOO is NOT set\n"); #!endif forward(); } ```

Currently this configuration will fail to load if environmental variable is not set. My proposal is to either allow `#!envdef` to accept an undefined environmental variable and have the preprocessor macro undefined, or to add a new keyword (`#!envdefn` perhaps?) to allow this behavior.