Make dependencies optional

array/meson.build

 subdir('src')
-subdir('test')
+
+if openmp_dep.found()
+    subdir('test')
+endif

meson.build

@@ -3,9 +3,9 @@ project('stdlib', 'c')
 cc = meson.get_compiler('c')
 libm = cc.find_library('m')
 
-glib_dep = dependency('glib-2.0', version: '>= 2.16')
-openmp_dep = dependency('openmp')
-thread_dep = dependency('threads')
+glib_dep = dependency('glib-2.0', version: '>= 2.16', required: false)
+openmp_dep = dependency('openmp', required: false)
+thread_dep = dependency('threads', required: false)
 
 incdir = include_directories('lib')
 
@@ -35,6 +35,12 @@ executable('write', 'write.c')
 subdir('array')
 subdir('hashtable')
 subdir('list')
-subdir('openmp')
-subdir('pthread')
 subdir('winter')
+
+if openmp_dep.found()
+    subdir('openmp')
+endif
+
+if thread_dep.found()
+    subdir('pthread')
+endif