[m-rev.] for review: Make link-time optimisation with clang work.

[Peter Wang]

configure.ac:
    Add -flto to LINK_SHARED_OBJ and LINK_SHARED_OBJ_SH commands
    when LTO is enabled with clang.

    Search for and use LTO-aware llvm-ar, llvm-ranlib and llvm-nm
    programs in place of the normal tools.

diff --git a/configure.ac b/configure.ac
index 6dbee8860..b27565dce 100644
--- a/configure.ac
+++ b/configure.ac
@@ -5361,8 +5361,17 @@ if test "$mercury_cv_lto" = "yes"; then
             NM=${GCC_NM:-$NM}
             ;;
         clang*)
+            # Also consider using flto=thin
             CFLAGS_FOR_LTO="-flto"
             LDFLAGS_FOR_LTO="-flto"
+            LINK_SHARED_OBJ="$LINK_SHARED_OBJ -flto"
+            LINK_SHARED_OBJ_SH="$LINK_SHARED_OBJ_SH -flto"
+            AC_CHECK_TOOL([LLVM_AR], [llvm-ar], [])
+            AC_CHECK_TOOL([LLVM_RANLIB], [llvm-ranlib], [])
+            AC_CHECK_TOOL([LLVM_NM], [llvm-nm], [])
+            AR=${LLVM_AR:-$AR}
+            RANLIB=${LLVM_RANLIB:-$RANLIB}
+            NM=${LLVM_NM:-$NM}
             ;;
         *)
             CFLAGS_FOR_LTO=""
-- 
2.25.0