COMP: Ensure CMAKE flags are passed down to all external project.

Due to Artichoke bug #6 [1], it is required to set the value in the cache
so that they can be considered when configuring the external projects.

This commit will fix the build error reported below:

// ----
/usr/bin/ld: /projects/pnl/home/petersv/testukf/build/lib/libitksys-4.6.a(SystemTools.cxx.o): relocation R_X86_64_32 against `.rodata.str1.1' can not be used when making a shared object; recompile with -fPIC
/projects/pnl/home/petersv/testukf/build/lib/libitksys-4.6.a: could not read symbols: Bad value
collect2: ld returned 1 exit status
// ----

[1] https://github.com/commontk/Artichoke/issues/6