Added simple test case for subproject. Now to make it pass...

test cases/common/49 subproject/meson.build

@@ -0,0 +1,9 @@
+project('subproj user', 'c')
+
+sub = subproject('sublib')
+
+inc = sub.get_variable('i')
+lib = sub.get_variable('l')
+
+e = executable('user.c', include_dirs : inc, link_with : lib)
+test('subdirtest', e)

test cases/common/49 subproject/sublib/include/subdefs.h

@@ -0,0 +1,6 @@
+#ifndef SUBDEFS_H_
+#define SUBDEFS_H_
+
+int subfunc();
+
+#endif

test cases/common/49 subproject/sublib/meson.build

@@ -0,0 +1,6 @@
+project('subproject', 'c')
+
+i = include_directories('include')
+l = shared_library('sublib', 'sublib.c', include_dirs : i, install : true)
+t = executable('simpletest', 'simpletest.c', include_dirs : i, link_with : l)
+test('plain', t)

test cases/common/49 subproject/sublib/simpletest.c

@@ -0,0 +1,5 @@
+#include<subdefs.h>
+
+int main(int argc, char **argv) {
+    return subfunc() == 42 ? 0 : 1;
+}

test cases/common/49 subproject/sublib/sublib.c

@@ -0,0 +1,5 @@
+#include<subdefs.h>
+
+int subfunc() {
+    return 42;
+}

test cases/common/49 subproject/user.c

@@ -0,0 +1,16 @@
+#include<subdefs.h>
+#include<stdio.h>
+
+
+int main(int argc, char **argv) {
+    int res;
+    printf("Calling into sublib now.\n");
+    res = subfunc();
+    if(res == 42) {
+        printf("Everything is fine.\n");
+        return 0;
+    } else {
+        printf("Something went wrong.\n");
+        return 1;
+    }
+}