Add a test case for a "library at project root" use case.

test cases/common/154 library at root/lib.c

@@ -0,0 +1,6 @@
+#if defined _WIN32 || defined __CYGWIN__
+__declspec(dllexport)
+#endif
+int fn(void) {
+    return -1;
+}

test cases/common/154 library at root/main/main.c

@@ -0,0 +1,5 @@
+extern int fn(void);
+
+int main() {
+    return 1 + fn();
+}

test cases/common/154 library at root/main/meson.build

@@ -0,0 +1,2 @@
+exe = executable('main', 'main.c', link_with : lib)
+test('stuff works', exe)

test cases/common/154 library at root/meson.build

@@ -0,0 +1,3 @@
+project('lib@root', 'c')
+lib = shared_library('lib', 'lib.c')
+subdir('main')