Fix pthread detection

configure.in always calls AC_CHECK_COMP_OPT(pthread) which succeeds with the
toolchain used although it does not contain thread support:

checking for mutex_lock in -lthread ... no
checking for pthread_mutex_lock in -lpthread... no
checking for pthread_mutex_lock in -lc... no
checking if compiler accepts -pthread... yes
checking for pthread_mutex_lock in -lc... no
checking for pthread_mutex_lock in -lpthread... no

This positive configure check leads to

    CFLAGS="$CFLAGS -pthread"

in ./configure, line 15757 which causes a compilation bug
/home/br/br4/output/host/opt/ext-toolchain/bin/../lib/gcc/arm-buildroot-linux-uclibcgnueabi/4.8.3/../../../../arm-buildroot-linux-uclibcgnueab

using this defconfig
http://autobuild.buildroot.net/results/e86/e86e97ff7887f1bdc6bf913498392ff50f56ea88/defconfig

This patch disables the freebsd-only code path. Patching configure directly
prevents the need for autoreconf.

Signed-off-by: Bernd Kuhls <bernd.kuhls@t-online.de>

diff -uNr unixODBC-2.3.2.org/configure unixODBC-2.3.2/configure
--- unixODBC-2.3.2.org/configure	2013-10-08 10:56:03.000000000 +0200
+++ unixODBC-2.3.2/configure	2015-03-13 21:19:59.210429211 +0100
@@ -15746,7 +15746,7 @@
 
     fi
 
-    if test "x$gotthread" = "xno"; then
+    if test "x$gotthread" = "xnot_needed_for_buildroot"; then
       if test "x$ac_cv_c_compiler_gnu" = "xyes"; then
                 { $as_echo "$as_me:${as_lineno-$LINENO}: checking if compiler accepts -pthread" >&5
 $as_echo_n "checking if compiler accepts -pthread... " >&6; }