meson: Leave dep_llvm empty if !with_llvm

The gallium auxiliary build would link against llvm, for the gallivm code
that it didn't build.  This broke the build on my armhf cross, where
libLLVM-3.9.so is not multiarch and thus points to x86-64 libs.

Reviewed-by: Dylan Baker <dylan@pnwbakers.com>
Tested-by: Timothy Arceri <tarceri@itsqueeze.com>
diff --git a/meson.build b/meson.build
index 314d15a..d8a8bb0 100644
--- a/meson.build
+++ b/meson.build
@@ -661,10 +661,11 @@
 if with_amd_vk
   llvm_modules += ['amdgpu', 'bitreader', 'ipo']
 endif
-dep_llvm = dependency(
-  'llvm', version : '>= 3.9.0', required : with_amd_vk, modules : llvm_modules,
-)
+dep_llvm = []
 if with_llvm
+  dep_llvm = dependency(
+    'llvm', version : '>= 3.9.0', required : with_amd_vk, modules : llvm_modules,
+  )
   if dep_llvm.found()
     _llvm_version = dep_llvm.version().split('.')
     # Development versions of LLVM have an 'svn' suffix, we don't want that for