cmake: Fix Windows static build

BUILD_SHARED_LIBS is a CMake variable. Due to how MSVC exports
and imports symbols from DLLs*, we still need to appropriately
define symbols so that __declspec(dllexport) and __declspec(dllimport)
are used properly, and also so that they're *not* used when building
statically, to avoid MSVC and LINK warnings LNK4217 and LNK4286,
and errors C2734, LNK2019, and LNK1120.

Do this by defining the compile-time definition ENABLE_STATIC
(not to be confused with the previous CMake variable ENABLE_STATIC)
whenever BUILD_SHARED_LIBS is set to OFF, and also defining
ENABLE_STATIC specifically for the static version of wsutil and
dumpcap / writecap that use it.

Also, eliminate some non-fatal warnings that occurred before by
defining WSUTIL_EXPORT to just be extern in the all static case.

Fixup 67bb36a181f9e643533bbfc96f52207edb47de0c

* - Data and functions are not directly exported, there's indirection
through an address table, but the appearance of direct export can be
obtained via syntactic sugar, See
https://learn.microsoft.com/en-us/cpp/build/importing-data-using-declspec-dllimport
https://learn.microsoft.com/en-us/cpp/c-language/rules-and-limitations-for-dllimport-dllexport
https://gcc.gnu.org/onlinedocs/gcc/Microsoft-Windows-Function-Attributes.html

CMakeLists.txt

@@ -541,6 +541,7 @@ set_property(DIRECTORY
 		$<$<OR:$<BOOL:${ENABLE_DEBUG}>,$<CONFIG:Debug>>:WS_DEBUG>
 		$<$<OR:$<AND:$<BOOL:${ENABLE_DEBUG}>,$<BOOL:${ENABLE_DEBUG_UTF_8}>>,$<CONFIG:Debug>>:WS_DEBUG_UTF_8>
 		$<$<BOOL:${ENABLE_ASSERT}>:ENABLE_ASSERT>
+		$<$<NOT:$<BOOL:${BUILD_SHARED_LIBS}>>:ENABLE_STATIC>
 )
 
 if(WIN32)
@@ -3497,7 +3498,7 @@ if(BUILD_dumpcap AND PCAP_FOUND)
 	#
 	# Instead, we just include source files for the relevant code
 	# to be built as part of dumpcap, so that they're built with
-	# BUILD_SHARED_LIBS defined, and will be able to be built with
+	# ENABLE_STATIC defined, and will be able to be built with
 	# the static version of libwsutil.
 	#
 	set(dumpcap_FILES
@@ -3515,7 +3516,7 @@ if(BUILD_dumpcap AND PCAP_FOUND)
 	set_extra_executable_properties(dumpcap "Executables")
 	target_link_libraries(dumpcap ${dumpcap_LIBS})
 	target_include_directories(dumpcap SYSTEM PRIVATE ${NL_INCLUDE_DIRS})
-	target_compile_definitions(dumpcap PRIVATE BUILD_SHARED_LIBS)
+	target_compile_definitions(dumpcap PRIVATE ENABLE_STATIC)
 	executable_link_mingw_unicode(dumpcap)
 	install(TARGETS dumpcap
 			RUNTIME	DESTINATION ${CMAKE_INSTALL_BINDIR}

include/ws_symbol_export.h

@@ -91,7 +91,7 @@
      * be necessary, e.g. if what's declared is an array whose size is
      * not given in the declaration.
      */
-    #ifdef BUILD_SHARED_LIBS
+    #ifdef ENABLE_STATIC
       /*
        * We're building all-static, so we're not building any DLLs.
        */
@@ -190,7 +190,14 @@
  * (not an executable) using MSVC.
  */
 #ifdef _MSC_VER
-#  ifdef BUILD_WSUTIL
+#  ifdef ENABLE_STATIC
+   /*
+    * We're building all-static, so we're not building any DLLs.
+    * Anything const and not initialized in the header (e.g., ws_utf8_seqlen)
+    * must be extern to avoid C2734.
+    */
+#    define WSUTIL_EXPORT  extern
+#  elif defined(BUILD_WSUTIL)
 #    define WSUTIL_EXPORT  __declspec(dllexport) extern
 #  else
 #    define WSUTIL_EXPORT  __declspec(dllimport) extern

writecap/CMakeLists.txt

@@ -37,7 +37,7 @@ set_target_properties(writecap PROPERTIES
 target_compile_definitions(writecap
 	PRIVATE
 		# Necessary to avoid C4273 inconsistent DLL linkage on MSVC
-		BUILD_SHARED_LIBS
+		ENABLE_STATIC
 )
 
 target_link_libraries(writecap

wsutil/CMakeLists.txt

@@ -431,7 +431,7 @@ add_library(wsutil_static STATIC
 )
 
 target_compile_definitions(wsutil_static PRIVATE
-	BUILD_SHARED_LIBS
+	ENABLE_STATIC
 	BUILD_WSUTIL
 )