build: propagate the static define for Swift

When building a Swift target against this library we need to ensure that
we pass along `-DCMARK_GFM_STATIC_DEFINE` to the Clang Importer. This is
required to build SwiftFormat against this library statically.

src/CMakeLists.txt

@@ -30,8 +30,12 @@ add_library(libcmark-gfm
  syntax_extension.c
  utf8.c
  xml.c)
-target_compile_definitions(libcmark-gfm PUBLIC
- $<$<NOT:$<BOOL:${BUILD_SHARED_LIBS}>>:CMARK_GFM_STATIC_DEFINE>)
+if(NOT BUILD_SHARED_LIBS)
+ target_compile_definitions(libcmark-gfm PUBLIC
+ CMARK_GFM_STATIC_DEFINE)
+ target_compile_options(libcmark-gfm PUBLIC
+ $<$<COMPILE_LANGUAGE:Swift>:-Xcc -DCMARK_GFM_STATIC_DEFINE>)
+endif()
 target_include_directories(libcmark-gfm PUBLIC
  $<BUILD_INTERFACE:${CMAKE_CURRENT_BINARY_DIR}>
  $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/src/include>