Support building mingw static and dynamic at the same time (#677)

Restrict the behavior disallowing the build of both libraries to MSVC. The mingw CMake generators
handle the situation properly:

- Shared library suffix: .dll
- Import library suffix: .a.dll
- Static library suffix: .a

CMakeLists.txt

@@ -804,7 +804,7 @@ else()
       draco_points_enc)
 
   # Library targets that consume the object collections.
-  if(MSVC OR WIN32)
+  if(MSVC)
     # In order to produce a DLL and import library the Windows tools require
     # that the exported symbols are part of the DLL target. The unfortunate side
     # effect of this is that a single configuration cannot output both the

cmake/draco_build_definitions.cmake

@@ -6,7 +6,7 @@ set(DRACO_CMAKE_DRACO_BUILD_DEFINITIONS_CMAKE_ 1)
 # Utility for controlling the main draco library dependency. This changes in
 # shared builds, and when an optional target requires a shared library build.
 macro(set_draco_target)
-  if(MSVC OR WIN32)
+  if(MSVC)
     set(draco_dependency draco)
     set(draco_plugin_dependency ${draco_dependency})
   else()

cmake/draco_install.cmake

@@ -55,7 +55,7 @@ macro(draco_setup_install_target)
   install(TARGETS draco_encoder DESTINATION
                   "${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_BINDIR}")
 
-  if(WIN32)
+  if(MSVC)
     install(TARGETS draco DESTINATION
                     "${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}")
   else()

cmake/draco_targets.cmake

@@ -164,8 +164,8 @@ endmacro()
 # cmake-format: off
 #   - OUTPUT_NAME: Override output file basename. Target basename defaults to
 #     NAME. OUTPUT_NAME is ignored when BUILD_SHARED_LIBS is enabled and CMake
-#     is generating a build for which MSVC or WIN32 are true. This is to avoid
-#     output basename collisions with DLL import libraries.
+#     is generating a build for which MSVC is true. This is to avoid output
+#     basename collisions with DLL import libraries.
 #   - TEST: Flag. Presence means treat library as a test.
 #   - DEFINES: List of preprocessor macro definitions.
 #   - INCLUDES: list of include directories for the target.
@@ -260,7 +260,7 @@ macro(draco_add_library)
   endif()
 
   if(lib_OUTPUT_NAME)
-    if(NOT (BUILD_SHARED_LIBS AND (MSVC OR WIN32)))
+    if(NOT (BUILD_SHARED_LIBS AND MSVC))
       set_target_properties(${lib_NAME}
                             PROPERTIES OUTPUT_NAME ${lib_OUTPUT_NAME})
     endif()