set a default buildtype in MesonNinja easyblock

[jfgrimm]

for e.g. CMake we add -DCMAKE_BUILD_TYPE=Release by default.
We don't currently do anything like that for meson: https://github.com/easybuilders/easybuild-easyblocks/blob/develop/easybuild/easyblocks/generic/mesonninja.py#L95

this means we actually default to debug without optimisations (ref):

Option	Default value	Description	Is per machine	Is per subproject
buildtype {plain, debug, debugoptimized, release, minsize, custom}	debug	Build type to use	no	no

these options correspond to (ref):

buildtype	debug	optimization
plain	false	plain
debug	true	0
debugoptimized	true	2
release	false	3
minsize	true	s

[Micket]

I wonder if CFLAGS are saving us here anyway? Of course we should still set this, but we might not be as bad as it seems in general :) I do hope it picks up -march flags in general as well, otherwise it would still be bad.
We do normally just go with -O2 with easybuild though, but i suspect differences are minimal.

Semi related: #3032 (comment)

[jfgrimm]

@Micket yes, it probably does. Better to be explicit though, rather than relying on what has priority internally in meson