build system: use llvm's lld linker when building with clang

Rakefile

@@ -138,6 +138,7 @@ def setup_globals
   cxxflags                += " #{c(:QT_CFLAGS)} #{c(:BOOST_CPPFLAGS)} #{c(:USER_CXXFLAGS)}"
 
   ldflags                  = ""
+  ldflags                 += " -fuse-ld=lld"                            if c?(:USE_CLANG) && !c(:LLVM_LLD).empty?
   ldflags                 += " -Llib/libebml/src -Llib/libmatroska/src" if c?(:EBML_MATROSKA_INTERNAL)
   ldflags                 += " #{c(:EXTRA_LDFLAGS)} #{c(:PROFILING_LIBS)} #{c(:USER_LDFLAGS)} #{c(:LDFLAGS_RPATHS)} #{c(:BOOST_LDFLAGS)}"
   ldflags                 += " -Wl,--dynamicbase,--nxcompat"               if $building_for[:windows]

ac/clang.m4

@@ -6,6 +6,11 @@ AC_DEFUN([AX_COMPILER_IS_CLANG],[
       ac_cv_compiler_is_clang=no
     fi
   ])
+
+  if test "x$ac_cv_compiler_is_clang" = xyes; then
+    AC_PATH_PROG(LLVM_LLD, "lld")
+  fi
+  AC_SUBST(LLVM_LLD)
 ])
 
 AX_COMPILER_IS_CLANG

build-config.in

@@ -93,6 +93,7 @@ LDFLAGS_RPATHS = @LDFLAGS_RPATHS@
 FLAC_LIBS = @FLAC_LIBS@
 ICONV_LIBS = @ICONV_LIBS@
 LIBINTL_LIBS = @LIBINTL_LIBS@
+LLVM_LLD = @LLVM_LLD@
 MAGIC_LIBS = @MAGIC_LIBS@
 MINGW_GUIAPP = @MINGW_GUIAPP@
 MINGW_LIBS = @MINGW_LIBS@