Add options instead of forcing them

[mauropasse]

When cross-compiling for RPi2, I got this error:

HEAD is now at c619113 Whitespace clean up (#70)
Submodule 'cmake/comp' (https://github.com/foonathan/compatibility.git) registered for path 'cmake/comp'
Cloning into '/root/ws/build/foonathan_memory_vendor/foo_mem-ext-prefix/src/foo_mem-ext/cmake/comp'...
/bin/sh: 1: ../tool/nodesize_dbg: not found
make[5]: *** [src/container_node_sizes_impl.hpp] Error 127
make[4]: *** [src/CMakeFiles/foonathan_memory.dir/all] Error 2
make[3]: *** [all] Error 2
make[2]: *** [foo_mem-ext-prefix/src/foo_mem-ext-stamp/foo_mem-ext-build] Error 2
make[1]: *** [CMakeFiles/foo_mem-ext.dir/all] Error 2
make: *** [all] Error 2
---
Failed   <<< foonathan_memory_vendor    [ Exited with code 2 ]

which lead me to add the option to not build the memory tools.

Signed-off-by: Mauro mpasserino@irobot.com

[MiguelCompany]

@mauropasse Thanks for this. I extended it for the other two options, so we can let the user of the vendor package what values to use for all the options.

[mauropasse]

Thanks @MiguelCompany !