Split executable modules from the library object, in particular this is
to prevent the exit() symbol to become part of the library, as this may
raise some flags in linting tools like "lintian" from Debian, e.g.:

X: libgrokj2k1: shlib-calls-exit usr/lib/x86_64-linux-gnu/libgrokj2k.so.7.1.0

Still keep executable sources grouped in a new GROK_EXECUTABLES_SRCS
variable to be able to run cppcheck on all of them.

The main() function is expected to be a global function, the code won't
link as a standalone executable if main() is put into a namespace.

This fixes the following linking error:

-----------------------------------------------------------------------
[ 32%] Linking CXX executable ../../../bin/test_sparse_array
/usr/bin/ld: /usr/lib/gcc/x86_64-linux-gnu/9/../../../x86_64-linux-gnu/Scrt1.o: in function `_start':
(.text+0x20): undefined reference to `main'
-----------------------------------------------------------------------

To ensure fidelity to original.