Problem: mingw static/shared builds have different preprocessor defines

Solution: remove objects optimisation in library build (similar to #2860)
and set PUBLIC compile definitions on all static builds instead of MSVC
only.

CMakeLists.txt

@@ -911,20 +911,27 @@ if (MSVC)
       DEBUG_POSTFIX "${MSVC_TOOLSET}-mt-sgd-${ZMQ_VERSION_MAJOR}_${ZMQ_VERSION_MINOR}_${ZMQ_VERSION_PATCH}"
       COMPILE_FLAGS "/DZMQ_STATIC"
       OUTPUT_NAME "libzmq")
-
-    target_compile_definitions(libzmq-static
-      PUBLIC ZMQ_STATIC
-      )
-
   endif()
 else ()
   # avoid building everything twice for shared + static
   # only on *nix, as Windows needs different preprocessor defines in static builds
-  add_library (objects OBJECT ${sources})
-  set_property(TARGET objects PROPERTY POSITION_INDEPENDENT_CODE ON)
+  if (NOT MINGW)
+    add_library (objects OBJECT ${sources})
+    set_property(TARGET objects PROPERTY POSITION_INDEPENDENT_CODE ON)
+    target_include_directories (objects
+      PUBLIC
+        $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
+        $<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}>
+        $<INSTALL_INTERFACE:include>
+    )
+  endif ()
 
   if (BUILD_SHARED)
-    add_library (libzmq SHARED $<TARGET_OBJECTS:objects> ${public_headers} ${html-docs} ${readme-docs} ${zmq-pkgconfig} ${CMAKE_CURRENT_BINARY_DIR}/version.rc)
+    if (MINGW)
+      add_library (libzmq SHARED ${sources} ${public_headers} ${html-docs} ${readme-docs} ${zmq-pkgconfig} ${CMAKE_CURRENT_BINARY_DIR}/version.rc)
+    else ()
+      add_library (libzmq SHARED $<TARGET_OBJECTS:objects> ${public_headers} ${html-docs} ${readme-docs} ${zmq-pkgconfig} ${CMAKE_CURRENT_BINARY_DIR}/version.rc)
+    endif ()
     # NOTE: the SOVERSION and VERSION MUST be the same as the one generated by libtool! It is NOT the same as the version of the package.
     set_target_properties (libzmq PROPERTIES
                           COMPILE_DEFINITIONS "DLL_EXPORT"
@@ -949,19 +956,22 @@ else ()
   endif()
 
   if (BUILD_STATIC)
-    add_library (libzmq-static STATIC $<TARGET_OBJECTS:objects> ${public_headers} ${html-docs} ${readme-docs} ${zmq-pkgconfig} ${CMAKE_CURRENT_BINARY_DIR}/version.rc)
+    if (MINGW)
+      add_library (libzmq-static STATIC ${sources} ${public_headers} ${html-docs} ${readme-docs} ${zmq-pkgconfig} ${CMAKE_CURRENT_BINARY_DIR}/version.rc)
+    else ()
+      add_library (libzmq-static STATIC $<TARGET_OBJECTS:objects> ${public_headers} ${html-docs} ${readme-docs} ${zmq-pkgconfig} ${CMAKE_CURRENT_BINARY_DIR}/version.rc)
+    endif ()
     set_target_properties (libzmq-static PROPERTIES
       PUBLIC_HEADER "${public_headers}"
-      COMPILE_DEFINITIONS "ZMQ_STATIC"
       OUTPUT_NAME "zmq"
       PREFIX "lib")
   endif()
 
-  target_include_directories (objects
-    PUBLIC
-      $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
-      $<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}>
-      $<INSTALL_INTERFACE:include>
+endif ()
+
+if (BUILD_STATIC)
+  target_compile_definitions(libzmq-static
+    PUBLIC ZMQ_STATIC
   )
 endif ()