cpnet: use gethostbyname_r() only when its declaration is known

[duncanmcqueen]

Split from #60.

Problem and fix

configure checks if gethostbyname_r() exists (HAVE_GETHOSTBYNAME_R), and then how many arguments it takes (HAVE_FUNC_GETHOSTBYNAME_R_6, _5 or _3). The second check reads the declaration in <netdb.h>.

On Haiku, libnetwork has gethostbyname_r(), but no header declares it. Thus the first check finds the function and the second check fails. Then cpnet.c stops with #error unknown number of arguments for gethostbyname_r.

Now cpnet.c uses gethostbyname_r() only if configure found the number of arguments. Otherwise it uses gethostbyname(), as it does when the function does not exist.

The first check finds the function only when libnetwork is in LIBS, which #73 does. Thus the error occurs on Haiku after #73, not with master alone. Please merge this pull request before #73 or together with it.

How to reproduce

On Haiku, with #73:

../configure --enable-jni ...
grep -n gethostbyname_r config.log
make

Results

Haiku R1/beta6 x86_64 (hrev59866), javac from OpenJDK 8, Kaffe 1.1.10-pre. I ran autoreconf on Linux (autoconf 2.73, automake 1.19) and built on Haiku out of tree with --enable-jni --disable-examples --disable-plugin --disable-gjdoc --disable-gtk-peer --disable-gconf-peer --disable-alsa --disable-dssi.

Before (master with #73):
checking for gethostbyname_r... yes
checking how many arguments gethostbyname_r() takes... cannot find function declaration in netdb.h
../../../../src/native/jni/native-lib/cpnet.c:656:4: error: #error unknown number of arguments for gethostbyname_r
make: *** [Makefile:617: cpnet.lo] Error 1

After (master with this pull request and #73):
cpnet.c compiles, and host name lookups work (see the NetBasics test in #73).

Platforms

Not specific to Haiku: it occurs on each system that has gethostbyname_r() in its libraries but does not declare it. Haiku is the system that I know. On Linux, configure gives checking for library containing inet_pton... none required and checking how many arguments gethostbyname_r() takes... six, so the result does not change on Linux.

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