[build] more portable autogen.sh shell script

Use more portable shell function definition, better supported by /bin/sh

Some /bin/sh, e.g. dash, do not support trap on ERR,
so that will issue some trace and will not trigger on ERR,
but the rest of the script runs fine.

autogen.sh

@@ -1,7 +1,7 @@
 #!/bin/sh
 # Run this to generate all the initial makefiles, etc.
 
-function errtrace {
+errtrace() {
    echo 1>&2 \
      "build requires autoconf automake libtool m4 pcre pcre-devel pkg-config"
 }