Fix zstd-dll build missing dependencies (#3496)

* Fixes zstd-dll build (https://github.com/facebook/zstd/issues/3492):
- Adds pool.o and threading.o dependency to the zstd-dll target
- Moves custom allocation functions into header to avoid needing to add dependency on common.o
- Adds test target for zstd-dll
- Adds github workflow that buildis zstd-dll

.github/workflows/dev-short-tests.yml

@@ -55,6 +55,14 @@ jobs:
       run: |
         make c89build V=1
 
+  build-zstd-dll:
+    runs-on: ubuntu-latest
+    steps:
+    - uses: actions/checkout@ac593985615ec2ede58e132d2e21d2b1cbd6127c # tag=v3
+    - name: build zstd bin against a dynamic lib (debuglevel for more dependencies)
+      run: |
+        make -C lib lib-mt-release 
+        DEBUGLEVEL=2 make -C programs zstd-dll
 
   gcc-7-libzstd:
     runs-on: ubuntu-latest
@@ -339,7 +347,6 @@ jobs:
         make -j -C programs allVariants MOREFLAGS=-O0
         ./tests/test-variants.sh
 
-
   qemu-consistency:
     name: QEMU ${{ matrix.name }}
     runs-on: ubuntu-20.04