Use ubuntu-18.04 for win32 CI builds

Signed-off-by: falkTX <falktx@falktx.com>

.github/workflows/example-plugins.yml

@@ -175,7 +175,7 @@ jobs:
             !bin/vst3
 
   win32:
-    runs-on: ubuntu-20.04
+    runs-on: ubuntu-18.04
     steps:
       - uses: actions/checkout@v2
         with:

Makefile.base.mk

@@ -170,7 +170,7 @@ endif
 
 ifeq ($(WINDOWS),true)
 # Assume we want posix
-BASE_FLAGS += -posix
+BASE_FLAGS += -posix -D__STDC_FORMAT_MACROS
 # Needed for windows, see https://github.com/falkTX/Carla/issues/855
 BASE_FLAGS += -mstackrealign
 else