Add detection for OpenBSD in configure script

Patch taken from downstream:
http://cvsweb.openbsd.org/cgi-bin/cvsweb/ports/net/qbittorrent/qbittorrent/patches/patch-configure_ac

configure

@@ -5200,6 +5200,18 @@ else $as_nop
 printf "%s\n" "no" >&6; }
 fi
 
+{ printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking whether OS is OpenBSD" >&5
+printf %s "checking whether OS is OpenBSD... " >&6; }
+if expr "$host_os" : ".*openbsd.*" > /dev/null
+then :
+  { printf "%s\n" "$as_me:${as_lineno-$LINENO}: result: yes" >&5
+printf "%s\n" "yes" >&6; }
+      LIBS="-lexecinfo $LIBS"
+else $as_nop
+  { printf "%s\n" "$as_me:${as_lineno-$LINENO}: result: no" >&5
+printf "%s\n" "no" >&6; }
+fi
+
 { printf "%s\n" "$as_me:${as_lineno-$LINENO}: checking whether OS is macOS" >&5
 printf %s "checking whether OS is macOS... " >&6; }
 if expr "$host_os" : ".*darwin.*" > /dev/null

configure.ac

@@ -60,6 +60,12 @@ AS_IF([expr "$host_os" : ".*freebsd.*" > /dev/null],
       LIBS="-lexecinfo $LIBS"],
       [AC_MSG_RESULT([no])])
 
+AC_MSG_CHECKING([whether OS is OpenBSD])
+AS_IF([expr "$host_os" : ".*openbsd.*" > /dev/null],
+      [AC_MSG_RESULT([yes])
+      LIBS="-lexecinfo $LIBS"],
+      [AC_MSG_RESULT([no])])
+
 AC_MSG_CHECKING([whether OS is macOS])
 AS_IF([expr "$host_os" : ".*darwin.*" > /dev/null],
       [AC_MSG_RESULT([yes])