Failure to build when `/etc/resolv.conf` is missing (e.g. with Mock)

[dvershinin]

In general, the build is 100% failing when /etc/resolv.conf is absent. See error below.

A specific way to reproduce this is to build a Tengine package with Mock.

Mock allows building RPMs in a clean environment. However, Tengine build fails because it relies on /etc/resolv.conf and the related define is not set because the file is not present:

In file included from src/core/ngx_core.h:55,
                 from src/core/ngx_resolver.c:9:
src/core/ngx_resolver.c: In function 'ngx_resolver_create':
src/core/ngx_resolver.c:283:41: error: 'NGX_RESOLVER_FILE' undeclared (first use in this function); did you mean 'T_NGX_RESOLVER_FILE'?
     ngx_str_t default_file = ngx_string(NGX_RESOLVER_FILE);
                                         ^~~~~~~~~~~~~~~~~
src/core/ngx_string.h:40:38: note: in definition of macro 'ngx_string'
 #define ngx_string(str)     { sizeof(str) - 1, (u_char *) str }
                                      ^~~
src/core/ngx_resolver.c:283:41: note: each undeclared identifier is reported only once for each function it appears in
     ngx_str_t default_file = ngx_string(NGX_RESOLVER_FILE);
                                         ^~~~~~~~~~~~~~~~~
src/core/ngx_string.h:40:38: note: in definition of macro 'ngx_string'
 #define ngx_string(str)     { sizeof(str) - 1, (u_char *) str }
                                      ^~~

How to reproduce it

1. install mock in an RPM distro
2. attempt to build RPM for Tengine
3. build always fails with error above because /etc/resolv.conf is not there, and thus NGX_RESOLVER_FILE is undefined

Alternative way reproduce it

1. Delete /etc/resolve.conf
2. attempt to build normally
3. build fails same as above

PR with fixes will follow.