Move LD definition to before libtool configuration. Also add GCC and LLVM support.

Don't see much difference in my simple tests..., LLVM support is not checked

configure.ac

@@ -38,13 +38,43 @@ AM_PROG_CC_C_O
 # Enable POSIX and platform extension preprocessor macros.
 AC_USE_SYSTEM_EXTENSIONS
 
-# Add libtool support.
-LT_INIT
-
 # Check for compiler version and vendor.
 AX_COMPILER_VENDOR
 AX_COMPILER_VERSION
 
+# Interprocedural optimization support. Needs special handling for linking and
+# archiving as well as compilation with Intels, needs to be done before
+# libtool is configured (to use correct LD).
+AC_ARG_ENABLE([ipo],
+   [AS_HELP_STRING([--enable-ipo],
+     [Enable interprocedural optimization @<:@no/yes@:>@]
+   )],
+   [enable_ipo="$enableval"],
+   [enable_ipo="no"]
+)
+
+if test "$enable_ipo" = "yes"; then
+   if test "$ax_cv_c_compiler_vendor" = "intel"; then
+      CFLAGS="$CFLAGS -ip -ipo"
+      LDFLAGS="$LDFLAGS -ipo"
+      : ${AR="xiar"}
+      : ${LD="xild"}
+      AC_MSG_RESULT([added Intel interprocedural optimization support])
+   elif test "$ax_cv_c_compiler_vendor" = "gnu"; then
+      CFLAGS="$CFLAGS -flto"
+      LDFLAGS="$LDFLAGS -flto"
+      AC_MSG_RESULT([added GCC interprocedural optimization support])
+   elif test "$ax_cv_c_compiler_vendor" = "clang"; then
+      CFLAGS="$CFLAGS -emit-llvm"
+      AC_MSG_RESULT([added LLVM interprocedural optimization support])
+   else
+      AC_MSG_WARN([Compiler does not support interprocedural optimization])
+   fi
+fi
+
+# Add libtool support.
+LT_INIT
+
 # Check for MPI. Need to do this before characterising the compiler (C99 mode),
 # as this changes the compiler.
 # We should consider using AX_PROG_CC_MPI to replace AC_PROG_CC when compiling
@@ -176,32 +206,12 @@ if test "$enable_san" = "yes"; then
    fi
    if test "$enable_san" = "yes"; then
       CFLAGS="$CFLAGS -fsanitize=address -fno-omit-frame-pointer"
+      AC_MSG_RESULT([added address sanitizer support])
    else
       AC_MSG_WARN([Compiler does not support address sanitizer option])
    fi
 fi
 
-# Intel compiler interprocedural optimization support. Needs special
-# handling for linking and archiving as well as compilation.
-AC_ARG_ENABLE([ipo],
-   [AS_HELP_STRING([--enable-ipo],
-     [Enable Intel interprocedural optimization @<:@no/yes@:>@]
-   )],
-   [enable_ipo="$enableval"],
-   [enable_ipo="no"]
-)
-
-if test "$enable_ipo" = "yes"; then
-   if test "$ax_cv_c_compiler_vendor" = "intel"; then
-      CFLAGS="$CFLAGS -ip -ipo"
-      LDFLAGS="-ipo"
-      AR="xiar"
-      LD="xild"
-   else
-      AC_MSG_WARN([Compiler does not support interprocedural optimization])
-   fi
-fi
-
 # Autoconf stuff.
 AC_PROG_INSTALL
 AC_PROG_MAKE_SET