package/mpdecimal: fix build without threads

Fix the following build failure raised since bump to version 2.5.1 in
commit 995c093d6a3e8042110d6a8d96fdc7ad65a557c4:

/home/peko/autobuild/instance-0/output-1/per-package/mpdecimal/host/opt/ext-toolchain/bin/../lib/gcc/arm-buildroot-linux-uclibcgnueabihf/9.3.0/../../../../arm-buildroot-linux-uclibcgnueabihf/bin/ld: cannot find -lpthread

Fixes:
 - http://autobuild.buildroot.org/results/b20c090f2ac5ea6d1c761820763037b87bde137a

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>

package/mpdecimal/mpdecimal.mk

@@ -11,6 +11,9 @@ MPDECIMAL_LICENSE = BSD-2-Clause
 MPDECIMAL_LICENSE_FILES = LICENSE.txt
 MPDECIMAL_CONF_OPTS = LD="$(TARGET_CC)"
 
+# threads are only needed for tests
+MPDECIMAL_MAKE_OPTS = MPD_PTHREAD=
+
 # On i386, by default, mpdecimal tries to uses <fenv.h> which is not
 # available in musl/uclibc. So in this case, we tell mpdecimal to use
 # the generic 32 bits code, which is anyway the one used on ARM,