plugins.example: Make installation relocatable

Fixes "make install" to be relocatable using DESTDIR. For that
we cannot use an absolute path as intallation directory target.
This is useful mostly to implement packaging using CPack.

It is a bit awkward to configure a default CMAKE_INSTALL_PREFIX
using WiresharkConfig.cmake but it seems to be working OK.

The previous non-relocatable behaviour may still be useful. It is
relegated to a custom "copy_plugin" target.

WiresharkConfig.cmake.in

@@ -4,9 +4,11 @@ set(Wireshark_PATCH_VERSION @PROJECT_PATCH_VERSION@)
 set(Wireshark_VERSION "@PROJECT_VERSION@")
 
 set(Wireshark_PLUGINS_ENABLED @HAVE_PLUGINS@)
+set(Wireshark_PLUGIN_LIBDIR "@PLUGIN_INSTALL_VERSION_LIBDIR@")
 
 @PACKAGE_INIT@
 
+set_and_check(Wireshark_INSTALL_PREFIX     "${PACKAGE_PREFIX_DIR}")
 set_and_check(Wireshark_LIB_DIR            "@PACKAGE_CMAKE_INSTALL_LIBDIR@")
 set_and_check(Wireshark_INCLUDE_DIR        "@PACKAGE_CMAKE_INSTALL_INCLUDEDIR@/wireshark")
 if(Wireshark_PLUGINS_ENABLED)

doc/plugins.example/CMakeLists.txt

@@ -14,6 +14,12 @@ project(Hello VERSION 0.0.1 DESCRIPTION "Wireshark Hello Plugin" LANGUAGES C)
 
 find_package(Wireshark CONFIG REQUIRED)
 
+if(CMAKE_INSTALL_PREFIX_INITIALIZED_TO_DEFAULT)
+	set(CMAKE_INSTALL_PREFIX "${Wireshark_INSTALL_PREFIX}"
+		CACHE PATH "Installation prefix" FORCE
+	)
+endif()
+
 if(NOT Wireshark_PLUGINS_ENABLED)
 	message(WARNING "Wireshark was compiled without support for plugins")
 endif()
@@ -36,5 +42,10 @@ set_target_properties(hello PROPERTIES PREFIX "" DEFINE_SYMBOL "")
 target_link_libraries(hello epan)
 
 install(TARGETS hello
-	LIBRARY DESTINATION "${Wireshark_PLUGIN_INSTALL_DIR}/epan" NAMELINK_SKIP
+	LIBRARY DESTINATION "${Wireshark_PLUGIN_LIBDIR}/epan" NAMELINK_SKIP
+)
+
+add_custom_target(copy_plugin
+	COMMAND ${CMAKE_COMMAND} -E copy $<TARGET_FILE:hello> ${Wireshark_PLUGIN_INSTALL_DIR}
+	COMMENT "Installing plugin to: ${Wireshark_PLUGIN_INSTALL_DIR}"
 )