Given the following configuration:

./configure \
 --with-shared-apache=../apache_1.3.6 \
 --with-xml=no \
 --enable-debug=no \
 --enable-safe-mode=yes \
 --enable-track-vars=yes \
 --enable-short-tags=no \
 --without-pcre-regex

during compilation I get the following error:
---------------------------------
        gcc -O2 -O2 -fpic  -I. -I.    -I../apache_1.3.6/src/include -I../apache_1.3.6/src/os/unix                                -c
functions/dns.c -o functions/dns.o
In file included from functions/dns.c:60:
/usr/local/lib/gcc-lib/powerpc-ibm-aix4.2.1.0/2.7.2.1/include/arpa/nameser.h:347: parse error before `_getshort'
/usr/local/lib/gcc-lib/powerpc-ibm-aix4.2.1.0/2.7.2.1/include/arpa/nameser.h:347: warning: data definition has no type or storage cl
ass
/usr/local/lib/gcc-lib/powerpc-ibm-aix4.2.1.0/2.7.2.1/include/arpa/nameser.h:348: parse error before `_getlong'
/usr/local/lib/gcc-lib/powerpc-ibm-aix4.2.1.0/2.7.2.1/include/arpa/nameser.h:348: warning: data definition has no type or storage cl
ass
In file included from functions/dns.c:61:
/usr/include/resolv.h:122: parse error before `u_int32_t'
/usr/include/resolv.h:122: warning: no semicolon at end of struct or union
/usr/include/resolv.h:122: warning: no semicolon at end of struct or union
/usr/include/resolv.h:123: warning: data definition has no type or storage class
/usr/include/resolv.h:125: parse error before `}'
/usr/include/resolv.h:260: parse error before `u_char'
/usr/include/resolv.h:261: parse error before `u_char'
/usr/include/resolv.h:263: warning: parameter names (without types) in function declaration
functions/dns.c: In function `php3_getmxrr':
functions/dns.c:313: `u_int16_t' undeclared (first use this function)
functions/dns.c:313: (Each undeclared identifier is reported only once
functions/dns.c:313: for each function it appears in.)
functions/dns.c:313: parse error before `t_cp'
functions/dns.c:313: parse error before `t_cp'
functions/dns.c:315: parse error before `t_cp'
functions/dns.c:315: parse error before `t_cp'
functions/dns.c:320: parse error before `t_cp'
functions/dns.c:320: parse error before `t_cp'
make: 1254-004 The error code from the last command is 1.
---------------------------------

I can fix this by adding the following:
#include <db.h>
to functions/dns.c, although I'm not positive this is the right solution.