grantlee: fix -lpthread build error

Fixing linking time error with certain toolchains. The issue caused by a missing explicit linking to
libpthread. The failed buildlog:
Linking CXX executable testgenerictypes_exec
/home/buildroot12git/output/host/usr/lib/gcc/arm-buildroot-linux-uclibcgnueabi/4.6.3/../../../../arm-buildroot-linux-uclibcgnueabi/bin/ld: /home/buildroot12git/output/host/usr/arm-buildroot-linux-uclibcgnueabi/sysroot/usr/lib/libQtCore.so: undefined reference to symbol 'pthread_cancel'
/home/buildroot12git/output/host/usr/lib/gcc/arm-buildroot-linux-uclibcgnueabi/4.6.3/../../../../arm-buildroot-linux-uclibcgnueabi/bin/ld: note: 'pthread_cancel' is defined in DSO /home/buildroot12git/output/host/usr/arm-buildroot-linux-uclibcgnueabi/sysroot/lib/libpthread.so.0 so try adding it to the linker command line
/home/buildroot12git/output/host/usr/arm-buildroot-linux-uclibcgnueabi/sysroot/lib/libpthread.so.0: could not read symbols: Invalid operation
collect2: ld returned 1 exit status
make[3]: *** [templates/tests/testgenerictypes_exec] Error 1
make[3]: Leaving directory `/home/buildroot12git/output/build/grantlee-0.2.0'
make[2]: *** [templates/tests/CMakeFiles/testgenerictypes_exec.dir/all] Error 2
make[2]: Leaving directory `/home/buildroot12git/output/build/grantlee-0.2.0'
make[1]: *** [all] Error 2
make[1]: Leaving directory `/home/buildroot12git/output/build/grantlee-0.2.0'
make: *** [/home/buildroot12git/output/build/grantlee-0.2.0/.stamp_built] Error

Signed-off-by: Zoltan Gyarmati <mr.zoltan.gyarmati@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>