Experiment with LTO for OS X and iOS, too

With Clang, LTO means compiling to LLVM intermediate representation
instead of object code, and optimisation and code generation at link
time.

As expected, linking the single executable for the iOS experimental
app takes quite some time (several hours), and in fact it eventually
fails. Hopefully better luck on OS X.

Need to use xcrun to find the right ar and ranlib, too, from the
Xcode toolchain being used.

Change-Id: Iee69397c57bf1d622882ad78c188e1734f6cbd63

configure.ac

@@ -850,9 +850,11 @@ AC_ARG_ENABLE(linkoo,
 AC_ARG_ENABLE(lto,
     AS_HELP_STRING([--enable-lto],
         [Enable link-time optimization. Suitable for product builds.
-         Building takes longer but libraries are optimized for speed.
-         (possible only with gcc-4.5 or later,
-          better to use gcc-4.6 and 'gold' as linker)]))
+         Building takes longer but libraries and executables are optimized for speed.
+         (For GCC, best to use GCC 4.6 or later and 'gold' linker.
+          Also possible for Clang.
+          Experimental work in progress,
+          don't use unless you are working on this.)]))
 
 AC_ARG_ENABLE(crashdump,
     AS_HELP_STRING([--enable-crashdump],
@@ -2384,6 +2386,18 @@ if test "$COMPATH" = "."; then
 fi
 COMPATH=`echo $COMPATH | $SED "s@/[[Bb]][[Ii]][[Nn]]\\\$@@"`
 
+# Set the ENABLE_LTO variable
+# ===================================================================
+AC_MSG_CHECKING([whether to use link-time optimization])
+if test -n "$enable_lto" -a "$enable_lto" != "no"; then
+    ENABLE_LTO="TRUE"
+    AC_MSG_RESULT([yes])
+else
+    ENABLE_LTO=""
+    AC_MSG_RESULT([no])
+fi
+AC_SUBST(ENABLE_LTO)
+
 dnl ===================================================================
 dnl  Test the Solaris compiler version
 dnl ===================================================================
@@ -2598,9 +2612,14 @@ if test $_os = Darwin; then
                 stdlib="-std=c++11 -stdlib=libc++"
                 CPP_LIBRARY="LIBCPP"
             fi
-            CC="`xcrun -find clang` $bitness -mmacosx-version-min=$with_macosx_version_min_required -isysroot $MACOSX_SDK_PATH"
-            CXX="`xcrun -find clang++` $bitness $stdlib -mmacosx-version-min=$with_macosx_version_min_required -isysroot $MACOSX_SDK_PATH"
+            if test "$ENABLE_LTO" = TRUE; then
+                lto=-flto
+            fi
+            CC="`xcrun -find clang` $bitness $lto -mmacosx-version-min=$with_macosx_version_min_required -isysroot $MACOSX_SDK_PATH"
+            CXX="`xcrun -find clang++` $bitness $lto $stdlib -mmacosx-version-min=$with_macosx_version_min_required -isysroot $MACOSX_SDK_PATH"
             XCRUN=xcrun
+            AR=`xcrun -find ar`
+            RANLIB=`xcrun -find ranlib`
             ;;
         esac
         AC_MSG_RESULT([$CC and $CXX])
@@ -2735,8 +2754,14 @@ if test $_os = iOS; then
 
     AC_MSG_RESULT($sysroot)
 
-    CXX="$xcode_developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang++ -arch $arch -isysroot $sysroot $versionmin"
-    CC="$xcode_developer/Toolchains/XcodeDefault.xctoolchain/usr/bin/clang -arch $arch -isysroot $sysroot $versionmin"
+    if test "$ENABLE_LTO" = TRUE; then
+        lto=-flto
+    fi
+    CC="`xcrun -find clang` -arch $arch -isysroot $sysroot $lto $versionmin"
+    CXX="`xcrun -find clang++` -arch $arch -isysroot $sysroot $lto $versionmin"
+    XCRUN=xcrun
+    AR=`xcrun -find ar`
+    RANLIB=`xcrun -find ranlib`
 fi
 
 AC_MSG_CHECKING([whether to treat the installation as read-only])
@@ -4333,18 +4358,6 @@ else
 fi
 AC_SUBST(DISABLE_LINKOO)
 
-# Set the ENABLE_LTO variable
-# ===================================================================
-AC_MSG_CHECKING([whether to use link-time optimization])
-if test -n "$enable_lto" -a "$enable_lto" != "no"; then
-    ENABLE_LTO="TRUE"
-    AC_MSG_RESULT([yes])
-else
-    ENABLE_LTO=""
-    AC_MSG_RESULT([no])
-fi
-AC_SUBST(ENABLE_LTO)
-
 if test "$enable_headless" = "yes"; then
     # be sure to do not mess with uneeded stuff
     test_randr=no