Add threads flag

This flag will toggle the use of `ghc-options` in the
`typed-process-test` test suite. It is set to `default: True`.

This is needed for tests to run on some GHC builds which do not support
a threaded runtime.

Closes: #59
Signed-off-by: hololeap <hololeap@users.noreply.github.com>

Author: hololeap

package.yaml

@@ -22,6 +22,12 @@ dependencies:
 - transformers
 - unliftio-core
 
+flags:
+  threads:
+    description: Use flags that depend on GHC supporting a threaded runtime (tests only)
+    default: True
+    manual: True
+
 library:
   source-dirs: src
   when:
@@ -32,10 +38,12 @@ tests:
   typed-process-test:
     main: Spec.hs
     source-dirs: test
-    ghc-options:
-    - -threaded
-    - -rtsopts
-    - -with-rtsopts=-N
+    when:
+    - condition: flag(threads)
+      ghc-options:
+      - -threaded
+      - -rtsopts
+      - -with-rtsopts=-N
     dependencies:
     - base64-bytestring
     - hspec