fix linking on OpenBSD

OpenBSD has a similar patch inside the ports, see

Allow KDE5 libraries to be built without having to link to libc explicitly.
Index: kde-modules/KDECompilerSettings.cmake
--- kde-modules/KDECompilerSettings.cmake.orig
+++ kde-modules/KDECompilerSettings.cmake
@@ -530,6 +530,8 @@ endfunction()
 # Better diagnostics (warnings, errors)
 ############################################################

+set(ALLOW_UNDEFINED_LIB_SYMBOLS No CACHE BOOL "allow undefined symbols in generated shared objects")
+
 if ((CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND NOT APPLE) OR
         (CMAKE_CXX_COMPILER_ID MATCHES "Clang" AND NOT APPLE) OR
         (CMAKE_CXX_COMPILER_ID STREQUAL "Intel" AND NOT WIN32))
@@ -537,9 +539,11 @@ if ((CMAKE_CXX_COMPILER_ID STREQUAL "GNU" AND NOT APPL
     set(CMAKE_SHARED_LINKER_FLAGS "-Wl,--fatal-warnings ${CMAKE_SHARED_LINKER_FLAGS}")
     set(CMAKE_MODULE_LINKER_FLAGS "-Wl,--fatal-warnings ${CMAKE_MODULE_LINKER_FLAGS}")

-    # Do not allow undefined symbols, even in non-symbolic shared libraries
-    set(CMAKE_SHARED_LINKER_FLAGS "-Wl,--no-undefined ${CMAKE_SHARED_LINKER_FLAGS}")
-    set(CMAKE_MODULE_LINKER_FLAGS "-Wl,--no-undefined ${CMAKE_MODULE_LINKER_FLAGS}")
+    if (NOT ${ALLOW_UNDEFINED_LIB_SYMBOLS})
+        # Do not allow undefined symbols, even in non-symbolic shared libraries
+        set(CMAKE_SHARED_LINKER_FLAGS "-Wl,--no-undefined ${CMAKE_SHARED_LINKER_FLAGS}")
+        set(CMAKE_MODULE_LINKER_FLAGS "-Wl,--no-undefined ${CMAKE_MODULE_LINKER_FLAGS}")
+    endif()
 endif()

 set(_KDE_GCC_COMMON_WARNING_FLAGS "-Wall -Wextra -Wcast-align -Wchar-subscripts -Wformat-security -Wno-long-long -Wpointer-arith -Wundef")