Fix OSS build

Summary:
There are two separate changes here.

### Use `find_package`

The old setup of "let's manually enumerate and order the libraries that Bistro depends on" worked fine, except:
 - it was a bit brittle (requiring occasional patches as deps changed), and
 - it garnered a lot of feedback to the effect of "your build is weird, so it's probably broken because of that."

Now I expect to have fewer breaks and more plausible deniability :)

 More importantly, this should make it much easier to migrate to `getdeps.py`.

## Statically link `fmt`

After `fmt` was added as a `folly` dependency, and linked into Folly code used by Bistro, its tests would fail to run with this error: `test_sqlite_task_store: error while loading shared libraries: libfmt.so.6: cannot open shared object file: No such file or directory`.

Something was getting messed up in the dynamic linking, and it wasn't clear to me what -- the way that Bistro is linking its dependencies certainly seems sensible. Most likely one of the dependencies is incompatible with dynamic linking in a subtle way. I suspect Proxygen.

The `fmt.py` change in this diff addresses this problem by forcing static linking on the offending library.

Reviewed By: yfeldblum

Differential Revision: D24604309

fbshipit-source-id: 35ecbbb277b25907ecaee493e8b0081d9f20b865

build/fbcode_builder/specs/fmt.py

@@ -8,6 +8,16 @@ from __future__ import unicode_literals
 
 def fbcode_builder_spec(builder):
     builder.add_option('fmtlib/fmt:git_hash', '6.2.1')
+    builder.add_option(
+        'fmtlib/fmt:cmake_defines',
+        {
+            # Avoids a bizarred failure to run tests in Bistro:
+            #   test_crontab_selector: error while loading shared libraries:
+            #   libfmt.so.6: cannot open shared object file:
+            #   No such file or directory
+            'BUILD_SHARED_LIBS': 'OFF',
+        }
+    )
     return {
         'steps': [
             builder.github_project_workdir('fmtlib/fmt', 'build'),

build/fbcode_builder/specs/folly.py

@@ -5,9 +5,12 @@ from __future__ import division
 from __future__ import print_function
 from __future__ import unicode_literals
 
+import specs.fmt as fmt
+
 
 def fbcode_builder_spec(builder):
     return {
+        "depends_on": [fmt],
         'steps': [
             # on macOS the filesystem is typically case insensitive.
             # We need to ensure that the CWD is not the folly source