build: garbage collect unused symbols by default

GCC can place each symbols into separate sections making it possible for
ld to discard unneeded sections (and thus symbols afterwards). This achieves
almost the same size benefits as LTO without the huge performance
penalty (and the undefined behavior with -g).

The cost of more section headers in .o files seems to be negligible.

Before:

  359.73user 71.94system 1:29.67elapsed 481%CPU (0avgtext+0avgdata 137948maxresident)k

     text        data     bss     dec     hex filename
  2772228       74848    9272 2856348  2b959c src/NetworkManager

With LTO:

  740.37user 94.89system 4:13.83elapsed 329%CPU (0avgtext+0avgdata 118316maxresident)k

     text        data     bss     dec     hex filename
  2435437       74224    8704 2518365  266d5d src/NetworkManager

With --gc-sections:

  365.63user 72.34system 1:30.12elapsed 485%CPU (0avgtext+0avgdata 138016maxresident)k

     text        data     bss     dec     hex filename
  2498498       74040    8408 2580946  2761d2 src/NetworkManager

configure.ac

@@ -935,6 +935,16 @@ else
     enable_lto='no'
 fi
 
+AC_ARG_ENABLE(ld-gc, AS_HELP_STRING([--enable-ld-gc], [Enable garbage collection of unused symbols on linking (default: auto)]))
+if (test "${enable_ld_gc}" != "no"); then
+    NM_COMPILER_FLAG([-fdata-sections -ffunction-sections -Wl,--gc-sections], [enable_ld_gc='yes'], [
+        if (test "${enable_ld_gc}" = "yes"); then
+            AC_MSG_ERROR([Unused symbol eviction requested but not supported.])
+        else
+            enable_ld_gc='no'
+        fi
+    ])
+fi
 
 dnl -------------------------
 dnl Vala bindings
@@ -1194,5 +1204,6 @@ echo "  more-asserts: $more_asserts"
 echo "  valgrind: $with_valgrind   $with_valgrind_suppressions"
 echo "  code coverage: $enable_code_coverage"
 echo "  LTO: $enable_lto"
+echo "  linker garbage collection: $enable_ld_gc"
 echo "  JSON validation: $enable_json_validation"
 echo