cmake: support the use of launchers in ctest -S scripts

[purpleKarrot]

When CTEST_USE_LAUNCHERS is set to ON in a ctest -S script, the configure step fails with the error message:

CMake Error:
  CTEST_USE_LAUNCHERS is enabled, but the RULE_LAUNCH_COMPILE global property
  is not defined.

  Did you forget to include(CTest) in the toplevel CMakeLists.txt ?

However, include(CTest) produces unwanted clutter. include(CTestUseLaunchers) is a more lightweight alternative.

To reproduce the issue, run the following script with and without the PR applied.

#!/usr/bin/env -S ctest -VV -S

set(CTEST_SOURCE_DIRECTORY "/path/to/secp256k1")
set(CTEST_BINARY_DIRECTORY "/path/to/secp256k1-build")

set(CTEST_CMAKE_GENERATOR "Ninja")
set(CTEST_USE_LAUNCHERS ON)

ctest_empty_binary_directory(${CTEST_BINARY_DIRECTORY})
ctest_start("Experimental")
ctest_configure()
ctest_build()

[real-or-random]

I think this deserves a comment. Otherwise, it looks like an unused import.

[real-or-random]

cc @hebasto