New cabal flag doctest to control building the doctest test-suite

`doctest-parallel` typically is not available for GHC prereleases.
Now the doctest suite switches itself off automatically if it cannot build.

regex-tdfa.cabal

@@ -58,6 +58,12 @@ flag force-O2
     .
     __NOTE__: This flag is mostly provided for legacy use-cases. Nowadays you can conveniently control optimization levels on a per-package granularity via @cabal.project@ files; see <https://cabal.readthedocs.io/en/latest/nix-local-build.html#configuring-builds-with-cabal-project cabal's user-guide> for more details.
 
+flag doctest
+  default: True
+  manual: False
+  description:
+    Include test-suite doctest.
+
 library
   hs-source-dirs:       lib
 
@@ -176,7 +182,7 @@ test-suite regex-tdfa-unittest
   if flag(force-O2)
     ghc-options:        -O2
 
-test-suite doc-test
+test-suite doctest
   type:           exitcode-stdio-1.0
   hs-source-dirs: test
   main-is:        DocTestMain.hs
@@ -188,3 +194,6 @@ test-suite doc-test
         -- doctest-parallel-0.2.2 is the first to filter out autogen-modules
 
   default-language:     Haskell2010
+
+  if !flag(doctest)
+    buildable: False