feat: Improve message associated with cython custom command

[jcfr]

Integrate changes originally introduced through:

• feat: rewrite #19

Before:

$ make
[...]
[ 33%] Generating C source simple.c
[ 66%] Building C object CMakeFiles/simple.dir/simple.c.o
[100%] Linking C shared module simple.cpython-39-x86_64-linux-gnu.so
[100%] Built target simple

After:

$ make
[...]
[ 33%] Generating C source 'simple.c' from 'simple.pyx'
[ 66%] Building C object CMakeFiles/simple.dir/simple.c.o
[100%] Linking C shared module simple.cpython-39-x86_64-linux-gnu.so
[100%] Built target simple