(build): update cxx/link flags for faster and better gdb experience

Description

Modify flags.cmake to add some flags:

CXX Debug

• -ggdb3 allows macro expansion and other useful things`
• -ggnu-pubnames generates data to ease gdb indexing by the linker`

Link

• -fuse-ld=gold will make use of gold linker, which is faster and allows --gdb-index
• -Wl,--gdb-index pass the --gdb-index option to the linker to generate indexes that will speedup gdb start

Closes #781 (closed)

How to test it?

Build everything (Debug + Release), launch all tests

Results:

Launch dicomXplorer with a break point in SView::starting() (Gcc):

• before: ~16s
• after: ~9s
• with -Og: ~8s

Launch SightViewer with a break point in SParameters::starting() (CLang):

• before: ~24s
• after (gold): ~9s
• after (ldd): ~9s