cmake: consolidate non-linux library handling

Move the handling to the top-level, adding it _once_ to EXTRA_LIBS.
Thus avoiding some duplication across the board.

Note that final handling varies a bit, namely:
 - OpenBSD/intl should be handled via the existing cmake/FindGettext.cmake
 - Darwin/resolv should not be needed since commit e98a32373 ("core: check
   if res_init requires linking with libresolv")
 - the backtrace/execinfo handling has been consolidated and moved

In the unlikely case of unwanted over-linking, the platforms can add
`-Wl,--as-needed` to their linker flags. Something which is strongly
encouraged and has been the default across multiple (linux) distros for
years.

Alternatively, if move quirks are needed they should be handled in a
single place.

Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>

CMakeLists.txt

@@ -311,6 +311,25 @@ else()
   add_custom_target(translations COMMAND true)
 endif()
 
+if(${CMAKE_SYSTEM_NAME} STREQUAL "FreeBSD")
+  find_library(EXECINFO_LIB_PATH execinfo /usr/local/lib)
+  set(CMAKE_REQUIRED_LIBRARIES "${EXECINFO_LIB_PATH}")
+  check_function_exists(backtrace HAVE_BACKTRACE)
+  list(APPEND EXTRA_LIBS "execinfo")
+else()
+  check_symbol_exists(backtrace "execinfo.h" HAVE_BACKTRACE)
+endif()
+
+if(${CMAKE_SYSTEM_NAME} STREQUAL "Haiku")
+  list(APPEND EXTRA_LIBS "network")
+else()
+  list(APPEND EXTRA_LIBS "pthread")
+endif()
+
+if(${CMAKE_SYSTEM_NAME} STREQUAL "SunOS")
+  list(APPEND EXTRA_LIBS "socket" "nsl")
+endif()
+
 list(APPEND EXTRA_LIBS "m")
 
 add_subdirectory(src)