[Noetic] zdepth_image_transport fails to build: fails to link against pthreads

[sloretz]

It looks like the package zdepth_image_transport is failing to build on all platforms, and is having trouble linking against pthreads

• https://build.ros.org/job/Nbin_uF64__zdepth_image_transport__ubuntu_focal_amd64__binary/32/consoleFull
• https://build.ros.org/job/Nbin_ufhf_uFhf__zdepth_image_transport__ubuntu_focal_armhf__binary/
• https://build.ros.org/job/Nbin_ufv8_uFv8__zdepth_image_transport__ubuntu_focal_arm64__binary/

23:04:13 Run Build Command(s):/usr/bin/make cmTC_79182/fast && make[2]: Entering directory '/tmp/binarydeb/ros-noetic-zdepth-image-transport-2.1.26/.obj-x86_64-linux-gnu/CMakeFiles/CMakeTmp'
23:04:13 /usr/bin/make -f CMakeFiles/cmTC_79182.dir/build.make CMakeFiles/cmTC_79182.dir/build
23:04:13 make[3]: Entering directory '/tmp/binarydeb/ros-noetic-zdepth-image-transport-2.1.26/.obj-x86_64-linux-gnu/CMakeFiles/CMakeTmp'
23:04:13 Building C object CMakeFiles/cmTC_79182.dir/CheckFunctionExists.c.o
23:04:13 /usr/lib/ccache/cc   -g -O2 -fdebug-prefix-map=/tmp/binarydeb/ros-noetic-zdepth-image-transport-2.1.26=. -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -DCHECK_FUNCTION_EXISTS=pthread_create   -o CMakeFiles/cmTC_79182.dir/CheckFunctionExists.c.o   -c /usr/share/cmake-3.16/Modules/CheckFunctionExists.c
23:04:13 Linking C executable cmTC_79182
23:04:13 /usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_79182.dir/link.txt --verbose=1
23:04:13 /usr/lib/ccache/cc -g -O2 -fdebug-prefix-map=/tmp/binarydeb/ros-noetic-zdepth-image-transport-2.1.26=. -fstack-protector-strong -Wformat -Werror=format-security -Wdate-time -D_FORTIFY_SOURCE=2 -DCHECK_FUNCTION_EXISTS=pthread_create    -rdynamic CMakeFiles/cmTC_79182.dir/CheckFunctionExists.c.o  -o cmTC_79182  -lpthreads 
23:04:13 /usr/bin/ld: cannot find -lpthreads
23:04:13 collect2: error: ld returned 1 exit status
23:04:13 make[3]: *** [CMakeFiles/cmTC_79182.dir/build.make:87: cmTC_79182] Error 1
23:04:13 make[3]: Leaving directory '/tmp/binarydeb/ros-noetic-zdepth-image-transport-2.1.26/.obj-x86_64-linux-gnu/CMakeFiles/CMakeTmp'
23:04:13 make[2]: *** [Makefile:121: cmTC_79182/fast] Error 2
23:04:13 make[2]: Leaving directory '/tmp/binarydeb/ros-noetic-zdepth-image-transport-2.1.26/.obj-x86_64-linux-gnu/CMakeFiles/CMakeTmp'