package/re2: needs threads

package/re2: needs threads

re2 unconditionally uses -pthread and add it to re2.pc

Moreover, it fails to buid without threads on:

In file included from /home/peko/autobuild/instance-0/output-1/per-package/re2/host/opt/ext-toolchain/arm-buildroot-linux-uclibcgnueabihf/include/c++/8.3.0/arm-buildroot-linux-uclibcgnueabihf/bits/os_defines.h:39,
                 from /home/peko/autobuild/instance-0/output-1/per-package/re2/host/opt/ext-toolchain/arm-buildroot-linux-uclibcgnueabihf/include/c++/8.3.0/arm-buildroot-linux-uclibcgnueabihf/bits/c++config.h:508,
                 from /home/peko/autobuild/instance-0/output-1/per-package/re2/host/opt/ext-toolchain/arm-buildroot-linux-uclibcgnueabihf/include/c++/8.3.0/bits/stl_algobase.h:59,
                 from /home/peko/autobuild/instance-0/output-1/per-package/re2/host/opt/ext-toolchain/arm-buildroot-linux-uclibcgnueabihf/include/c++/8.3.0/memory:62,
                 from /home/peko/autobuild/instance-0/output-1/build/re2-2020-08-01/re2/filtered_re2.h:24,
                 from /home/peko/autobuild/instance-0/output-1/build/re2-2020-08-01/re2/filtered_re2.cc:5:
/home/peko/autobuild/instance-0/output-1/per-package/re2/host/arm-buildroot-linux-uclibcgnueabihf/sysroot/usr/include/features.h:218:5: warning: #warning requested reentrant code, but thread support was disabled [-Wcpp]
 #   warning requested reentrant code, but thread support was disabled
     ^~~~~~~
In file included from /home/peko/autobuild/instance-0/output-1/build/re2-2020-08-01/re2/prog.h:22,
                 from /home/peko/autobuild/instance-0/output-1/build/re2-2020-08-01/re2/bitstate.cc:28:
/home/peko/autobuild/instance-0/output-1/build/re2-2020-08-01/re2/re2.h:768:16: error: 'once_flag' in namespace 'std' does not name a type
   mutable std::once_flag rprog_once_;
                ^~~~~~~~~
/home/peko/autobuild/instance-0/output-1/build/re2-2020-08-01/re2/re2.h:768:11: note: 'std::once_flag' is defined in header '<mutex>'; did you forget to '#include <mutex>'?
/home/peko/autobuild/instance-0/output-1/build/re2-2020-08-01/re2/re2.h:218:1:
+#include <mutex>

Fixes:
 - http://autobuild.buildroot.org/results/7d7c6dcac3cb8ea6deb753178e85eb1c5c74c8e3

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Michael Nosthoff <buildroot@heine.tech>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>