configure: add LTO optarg

This allows users to specify an argument such as './configure
--enable-lto=thin' to use ThinLTO with Clang. The old functionality with
'./configure --enable-lto' is preserved.

Signed-off-by: J. Dekker <jdek@itanimul.li>

configure

@@ -412,7 +412,7 @@ Toolchain options:
   --build-suffix=SUFFIX    library name suffix []
   --enable-pic             build position-independent code
   --enable-thumb           compile for Thumb instruction set
-  --enable-lto             use link-time optimization
+  --enable-lto[=arg]       use link-time optimization
   --env="ENV=override"     override the environment variables
 
 Advanced options (experts only):
@@ -2524,7 +2524,6 @@ CMDLINE_SELECT="
     debug
     extra_warnings
     logging
-    lto
     optimizations
     rpath
     stripping
@@ -4170,6 +4169,9 @@ for opt do
         --enable-sdl)
             enable sdl2
         ;;
+        --enable-lto*)
+            lto=-f${opt#--enable-}
+        ;;
         --enable-*=*|--disable-*=*)
             eval $(echo "${opt%%=*}" | sed 's/--/action=/;s/-/ thing=/')
             is_in "${thing}s" $COMPONENT_LIST || die_unknown "$opt"
@@ -4639,7 +4641,7 @@ icl_flags(){
             # on Windows, does enable remarks so disable them here.
             -Wall)                echo $flag -Qdiag-disable:remark ;;
             -std=c99)             echo -Qstd=c99 ;;
-            -flto)                echo -ipo ;;
+            -flto*)               echo -ipo ;;
         esac
     done
 }
@@ -4647,7 +4649,7 @@ icl_flags(){
 icc_flags(){
     for flag; do
         case $flag in
-            -flto)                echo -ipo ;;
+            -flto*)               echo -ipo ;;
             *)                    echo $flag ;;
         esac
     done
@@ -7182,17 +7184,17 @@ fi
 
 check_optflags(){
     check_cflags "$@"
-    enabled lto && check_ldflags "$@"
+    [ -n "$lto" ] && check_ldflags "$@"
 }
 
 check_optflags $optflags
 check_optflags -fno-math-errno
 check_optflags -fno-signed-zeros
 
-if enabled lto; then
+if [ -n "$lto" ]; then
     test "$cc_type" != "$ld_type" && die "LTO requires same compiler and linker"
-    check_cflags  -flto
-    check_ldflags -flto $cpuflags
+    check_cflags  $lto
+    check_ldflags $lto $cpuflags
     disable inline_asm_direct_symbol_refs
 fi
 
@@ -7223,7 +7225,7 @@ if enabled icc; then
     # icc 11.0 and 11.1 work with ebp_available, but don't pass the test
     enable ebp_available
     # The test above does not test linking
-    enabled lto && disable symver_asm_label
+    [ -n "$lto" ] && disable symver_asm_label
     if enabled x86_32; then
         icc_version=$($cc -dumpversion)
         test ${icc_version%%.*} -ge 11 &&