Determining if the function openpty exists failed with the following output:
Change Dir: /home/dane/devel/aur/qgis-community/src/qgis-3.0.3/build/CMakeFiles/CMakeTmp

Run Build Command:"/usr/bin/make" "cmTC_ac109/fast"
/usr/bin/make -f CMakeFiles/cmTC_ac109.dir/build.make CMakeFiles/cmTC_ac109.dir/build
make[1]: Entering directory '/home/dane/devel/aur/qgis-community/src/qgis-3.0.3/build/CMakeFiles/CMakeTmp'
Building C object CMakeFiles/cmTC_ac109.dir/CheckFunctionExists.c.o
/usr/bin/cc   -march=native -O2 -pipe -fstack-protector-strong -fno-plt -DCHECK_FUNCTION_EXISTS=openpty   -o CMakeFiles/cmTC_ac109.dir/CheckFunctionExists.c.o   -c /usr/share/cmake-3.11/Modules/CheckFunctionExists.c
Linking C executable cmTC_ac109
/usr/bin/cmake -E cmake_link_script CMakeFiles/cmTC_ac109.dir/link.txt --verbose=1
/usr/bin/cc -march=native -O2 -pipe -fstack-protector-strong -fno-plt -DCHECK_FUNCTION_EXISTS=openpty  -Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now  -rdynamic CMakeFiles/cmTC_ac109.dir/CheckFunctionExists.c.o  -o cmTC_ac109 
CMakeFiles/cmTC_ac109.dir/CheckFunctionExists.c.o: In function `main':
CheckFunctionExists.c:(.text.startup+0xd): undefined reference to `openpty'
collect2: error: ld returned 1 exit status
make[1]: *** [CMakeFiles/cmTC_ac109.dir/build.make:87: cmTC_ac109] Error 1
make[1]: Leaving directory '/home/dane/devel/aur/qgis-community/src/qgis-3.0.3/build/CMakeFiles/CMakeTmp'
make: *** [Makefile:126: cmTC_ac109/fast] Error 2