dbus: use -pthread instead of -lpthread

This fixes a problem in other packages when they link against dbus. For
instance, with the dnsmasq package:

/buildroot/output/host/usr/bin/mipsel-ctng-linux-uclibc-gcc --static
-lidn  -o dnsmasq cache.o rfc1035.o util.o option.o forward.o network.o
dnsmasq.o dhcp.o lease.o rfc2131.o netlink.o dbus.o bpf.o helper.o
tftp.o log.o conntrack.o dhcp6.o rfc3315.o dhcp-common.o outpacket.o
radv.o slaac.o auth.o ipset.o domain.o dnssec.o blockdata.o
-L/buildroot/output/host/usr/mipsel-buildroot-linux-uclibc/sysroot/usr/lib
-ldbus-1 -lpthread
-L/buildroot/output/host/usr/mipsel-buildroot-linux-uclibc/sysroot/usr/lib
-lidn -liconv
-L/buildroot/output/host/usr/mipsel-buildroot-linux-uclibc/sysroot/usr/lib
-llua -lm     -Wl,-Bstatic -lhogweed
-L/buildroot/output/host/usr/mipsel-buildroot-linux-uclibc/sysroot/usr/lib
-lnettle -lgmp   -Wl,-Bdynamic -Wl,-Bstatic -lgmp -Wl,-Bdynamic
/buildroot/output/host/usr/mipsel-buildroot-linux-uclibc/sysroot/usr/lib/libpthread.a(init.os):
In function `__pthread_initialize_minimal_internal':
init.c:(.text+0x26c): undefined reference to `__libc_setup_tls'
collect2: error: ld returned 1 exit status
make[2]: *** [dnsmasq] Error 1
make[2]: Leaving directory
`/buildroot/output/build/dnsmasq-2.71/src'

When dnsmasq tries to link against dbus it checks the dbus pkgconfig
file to obtain the LIBS that it need to pass to the linker. In the dbus
.pc file you have "-ldbus-1 -lpthread" because -lpthread was used when
building dbus. If you use -pthread when you build dbus, then you will
have -pthread in the dbus .pc file as well.

-lpthread means that we want to link with a library called pthread.
-pthread is a compiler option which will do whatever is needed to be
done to compile with the pthreads standards so that it works.

Fixes:
  http://autobuild.buildroot.net/results/7cd/7cdab3b6f74dbfde965300a51e58a377ba30602b/

Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>

package/dbus/dbus.mk

@@ -35,7 +35,7 @@ define DBUS_USERS
 endef
 
 ifeq ($(BR2_PREFER_STATIC_LIB),y)
-DBUS_CONF_OPT += LIBS='-lpthread'
+DBUS_CONF_OPT += LIBS='-pthread'
 endif
 
 ifeq ($(BR2_microblaze),y)