cmake: bail out if the source tree has been configured...

cmake: bail out if the source tree has been configured...

.. and print a helpful message.

Before:

./configure
mkdir ../cln_build
cd ../cln_build && cmake -GNinja ../cln && cmake --build .

[cryptic error due to a redefined macro or something]

After:

./configure
mkdir ../cln_build
cd ../cln_build && cmake -GNinja ../cln && cmake --build .

-- The C compiler identification is GNU 10.2.0
-- The CXX compiler identification is GNU 10.2.0
-- Check for working C compiler: /usr/bin/cc
-- Check for working C compiler: /usr/bin/cc -- works
-- Detecting C compiler ABI info
-- Detecting C compiler ABI info - done
-- Detecting C compile features
-- Detecting C compile features - done
-- Check for working CXX compiler: /usr/bin/c++
-- Check for working CXX compiler: /usr/bin/c++ -- works
-- Detecting CXX compiler ABI info
-- Detecting CXX compiler ABI info - done
-- Detecting CXX compile features
-- Detecting CXX compile features - done
CMake Error at CMakeLists.txt:24 (message):
  Source directory has been already configured by configure script. Please
  run `make distclean` in /home/asheplyakov/work/sw/cln and try again.

-- Configuring incomplete, errors occurred!