openssl: only build threads configuration if toolchain has pthread support

Gets rid of a large number of warnings (and suboptimal code?):

..sysroot/usr/include/features.h:209:5: warning: #warning requested
reentrant code, but thread support was disabled [-Wcpp]

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/openssl/openssl.mk

@@ -73,7 +73,7 @@ define OPENSSL_CONFIGURE_CMDS
 			--prefix=/usr \
 			--openssldir=/etc/ssl \
 			--libdir=/lib \
-			threads \
+			$(if $(BR2_TOOLCHAIN_HAS_THREADS),threads,no-threads) \
 			$(if $(BR2_PREFER_STATIC_LIB),no-shared,shared) \
 			no-idea \
 			no-rc5 \