Add requirement for POSIX threads (pthreads)

I wrote this one-liner, and then realized I couldn't test it - I have
no systems that do not have pthreads as an integral part - does anybody?
Since <pthread.h> has been a de facto requirement since 4.5, the point may
be moot.
(Update: quick '# mv pthread.h pthread2.h' and it fails)

And are the URLs in README.developer correct? Should they point to github?

README.developer

@@ -159,7 +159,7 @@ file.  This document introduces hacking the code of Hamlib.
 
   git clone git://git.code.sf.net/p/hamlib/code hamlib
 
-The clone has to only be done the first time.
+The clone only has to be done the first time.
 
 After the initial clone, whenever you want to update your local
 repository, issue the following command in the root directory of Hamlib:
@@ -277,6 +277,9 @@ distributions may differ).
 * libtool       2.2.6b+                     # libtool --version
 * Git for connection to git.code.sf.net/p/hamlib/code
 
+As of Hamlib 4.7, POSIX thread support (pthreads) is required to compile or
+run Hamlib.
+
 N.B. Hamlib requires libtool >= 2.2.6b in compliance with CVE-2009-3736.
 
 Optional, but highly recommended:
@@ -518,6 +521,7 @@ So far, Hamlib has been tested successfully under the following systems:
     * Debian sid mipsel
     * Raspbian armhf (Raspberry Pi Debian derivative)
     * RedHat i386
+    * openSUSE (Leap & Tumbleweed)
     * Linux ppc
     * Slackware i386
     * FreeBSD & NetBSD

configure.ac

@@ -172,7 +172,7 @@ dnl If pthread support is found, this macro defines HAVE_PTHREAD and
 dnl AC_SUBST's PTHREAD_LIBS, PTHREAD_CFLAGS, and PTHREAD_CC making those
 dnl variables available in Makefiles.
 # macros/ax_pthread.m4
-AX_PTHREAD
+AX_PTHREAD([], [AC_MSG_ERROR([POSIX threads not found])])
 
 AC_SYS_POSIX_TERMIOS()