Determining if the function smtp_create_session exists in the esmtp failed with the following output:
Change Dir: /home/buildroot/autobuild/instance-0/output-1/build/log4cxx-0.12.0/CMakeFiles/CMakeTmp

Run Build Command(s):/usr/bin/make cmTC_f6b9d/fast && make[1]: Entering directory '/home/buildroot/autobuild/instance-0/output-1/build/log4cxx-0.12.0/CMakeFiles/CMakeTmp'
/usr/bin/make  -f CMakeFiles/cmTC_f6b9d.dir/build.make CMakeFiles/cmTC_f6b9d.dir/build
make[2]: Entering directory '/home/buildroot/autobuild/instance-0/output-1/build/log4cxx-0.12.0/CMakeFiles/CMakeTmp'
Building CXX object CMakeFiles/cmTC_f6b9d.dir/CheckFunctionExists.cxx.o
/home/buildroot/autobuild/instance-0/output-1/host/bin/x86_64-linux-g++ --sysroot=/home/buildroot/autobuild/instance-0/output-1/host/x86_64-buildroot-linux-musl/sysroot    -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -g0 -DCHECK_FUNCTION_EXISTS=smtp_create_session  -DNDEBUG   -std=c++17 -o CMakeFiles/cmTC_f6b9d.dir/CheckFunctionExists.cxx.o -c /home/buildroot/autobuild/instance-0/output-1/build/log4cxx-0.12.0/CMakeFiles/CheckLibraryExists/CheckFunctionExists.cxx
Linking CXX executable cmTC_f6b9d
/home/buildroot/autobuild/instance-0/output-1/host/bin/cmake -E cmake_link_script CMakeFiles/cmTC_f6b9d.dir/link.txt --verbose=1
/home/buildroot/autobuild/instance-0/output-1/host/bin/x86_64-linux-g++ --sysroot=/home/buildroot/autobuild/instance-0/output-1/host/x86_64-buildroot-linux-musl/sysroot  -D_LARGEFILE_SOURCE -D_LARGEFILE64_SOURCE -D_FILE_OFFSET_BITS=64 -Os -g0 -DCHECK_FUNCTION_EXISTS=smtp_create_session  -DNDEBUG    CMakeFiles/cmTC_f6b9d.dir/CheckFunctionExists.cxx.o  -o cmTC_f6b9d  -lesmtp 
/home/buildroot/autobuild/instance-0/output-1/host/opt/ext-toolchain/bin/../lib/gcc/x86_64-buildroot-linux-musl/9.3.0/../../../../x86_64-buildroot-linux-musl/bin/ld: cannot find -lesmtp
collect2: error: ld returned 1 exit status
CMakeFiles/cmTC_f6b9d.dir/build.make:89: recipe for target 'cmTC_f6b9d' failed
make[2]: *** [cmTC_f6b9d] Error 1
make[2]: Leaving directory '/home/buildroot/autobuild/instance-0/output-1/build/log4cxx-0.12.0/CMakeFiles/CMakeTmp'
Makefile:124: recipe for target 'cmTC_f6b9d/fast' failed
make[1]: *** [cmTC_f6b9d/fast] Error 2
make[1]: Leaving directory '/home/buildroot/autobuild/instance-0/output-1/build/log4cxx-0.12.0/CMakeFiles/CMakeTmp'