configure: add --enable-lto option

This works with gcc.  Other compilers might need to have a flag
mapping added.

Signed-off-by: Mans Rullgard <mans@mansr.com>

configure

@@ -238,6 +238,7 @@ Advanced options (experts only):
                            disable buffer boundary checking in bitreaders
                            (faster, but may crash)
   --enable-memalign-hack   emulate memalign, interferes with memory debuggers
+  --enable-lto             use link-time optimization
 
 Optimization options (experts only):
   --disable-asm            disable all assembler optimizations
@@ -1290,6 +1291,7 @@ CMDLINE_SELECT="
     debug
     extra_warnings
     logging
+    lto
     optimizations
 "
 
@@ -3445,17 +3447,31 @@ void ff_foo(void) {}
 EOF
 fi
 
-if [ -n "$optflags" ]; then
-    add_cflags $optflags
-elif enabled small; then
-    add_cflags $cflags_size
-elif enabled optimizations; then
-    add_cflags $cflags_speed
-else
-    add_cflags $cflags_noopt
+if [ -z "$optflags" ]; then
+    if enabled small; then
+        optflags=$cflags_size
+    elif enabled optimizations; then
+        optflags=$cflags_speed
+    else
+        optflags=$cflags_noopt
+    fi
 fi
-check_cflags -fno-math-errno
-check_cflags -fno-signed-zeros
+
+check_optflags(){
+    check_cflags "$@"
+    enabled lto && check_ldflags "$@"
+}
+
+
+if enabled lto; then
+    test "$cc_type" != "$ld_type" && die "LTO requires same compiler and linker"
+    check_cflags  -flto
+    check_ldflags -flto $cpuflags
+fi
+
+check_optflags $optflags
+check_optflags -fno-math-errno
+check_optflags -fno-signed-zeros
 
 if enabled icc; then
     # Just warnings, no remarks
@@ -3490,7 +3506,7 @@ elif enabled ccc; then
     add_cflags -msg_disable ptrmismatch1
     add_cflags -msg_disable unreachcode
 elif enabled gcc; then
-    check_cflags -fno-tree-vectorize
+    check_optflags -fno-tree-vectorize
     check_cflags -Werror=implicit-function-declaration
     check_cflags -Werror=missing-prototypes
     check_cflags -Werror=declaration-after-statement