meson: do not search for needless deps

If we don't want to use these deps, there's no good reason to search
for them in the first place. This should shave a bit of time for the
initial build.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>

meson_options.txt

@@ -138,15 +138,17 @@ option(
 )
 option(
   'valgrind',
-  type : 'boolean',
-  value : true,
-  description : 'Build with valgrind support if possible'
+  type : 'combo',
+  value : 'auto',
+  choices : ['auto', 'true', 'false'],
+  description : 'Build with valgrind support'
 )
 option(
   'libunwind',
-  type : 'boolean',
-  value : true,
-  description : 'Use libunwind for stack-traces if possible'
+  type : 'combo',
+  value : 'auto',
+  choices : ['auto', 'true', 'false'],
+  description : 'Use libunwind for stack-traces'
 )
 option(
   'build-tests',