build: Make libSDL2 optional and only link orblcd against it

* Only orblcd requires libSDL2.
* Allow building the command-line tools if libSDL2 is not found.
* Don't link them all against libSDL2.

meson.build

@@ -13,7 +13,6 @@ dependencies = [
     dependency('threads'),
     dependency('libusb-1.0'),
     dependency('libzmq'),
-    dependency('SDL2'),
     dependency('ncurses', 'ncursesw'),
     dependency('libelf'),
     uicolours_default,
@@ -51,6 +50,11 @@ if not libcapstone.found()
     libcapstone = disabler()
 endif
 
+libSDL2 = dependency('SDL2', required: false)
+if not libSDL2.found()
+    libSDL2 = disabler()
+endif
+
 if host_machine.system() == 'windows'
     stream_src = [
         'Src/stream_win32.c',
@@ -227,7 +231,9 @@ executable('orblcd',
         git_version_info_h,
     ],
     include_directories: incdirs,
-    dependencies: dependencies,
+    dependencies: dependencies + [
+        libSDL2,
+    ],
     link_with: liborb,
     install: true,
 )