doc: update OpenBSD build instructions

Author: hubslave

doc/openbsd.txt

@@ -1,13 +1,12 @@
+In addition to the dependencies listed in the readme, the following packages
+are required:
 
-This is one user's ./configure invocation, that worked on OpenBSD.
+	autoconf & automake (for the build system)
+	gmake (the Makefiles generated by Autotools aren't BSD make compatible)
+	argp-standalone (OpenBSD's libc lacks the argp_* interfaces)
 
-Certainly the pathnames may differ, but the general idea is clear:  Set
-CFLAGS, LDFLAGS, and other environment vars as needed, prior to
-./configure invocation.
+After that, the standard autogen-configure-build sequence should work:
 
-AUTOMAKE_VERSION=1.11 AUTOCONF_VERSION=2.68 ./autogen.sh && \
-CC=clang LD=llvm-ld CFLAGS='-O2 -Wall -g -I/usr/local/include/event2 \
--I/opt/OpenBSD/5.1/amd64/jansson-2.4/include -I/usr/local/include/event2' \
- LDFLAGS='-L/opt/OpenBSD/5.1/amd64/jansson-2.4/lib -L/usr/local/lib -L/usr/lib' \
-      ./configure --prefix=/opt/OpenBSD/5.1/amd64/picocoin-0.0 && \
-      make all check
+	AUTOMAKE_VERSION=1.15 AUTOCONF_VERSION=2.69 ./autogen.sh
+	./configure CPPFLAGS=-I/usr/local/include LDFLAGS=-L/usr/local/lib
+	gmake all check