configure,meson: add option to enable LTO

This patch allows to compile QEMU with link-time optimization (LTO).
Compilation with LTO is handled directly by meson. This patch only
adds the option in configure and forwards the request to meson

Tested with all major versions of clang from 6 to 12

Signed-off-by: Daniele Buono <dbuono@linux.vnet.ibm.com>
Message-Id: <20201204230615.2392-2-dbuono@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

configure

@@ -242,6 +242,7 @@ host_cc="cc"
 audio_win_int=""
 libs_qga=""
 debug_info="yes"
+lto="false"
 stack_protector=""
 safe_stack=""
 use_containers="yes"
@@ -1182,6 +1183,10 @@ for opt do
   ;;
   --disable-werror) werror="no"
   ;;
+  --enable-lto) lto="true"
+  ;;
+  --disable-lto) lto="false"
+  ;;
   --enable-stack-protector) stack_protector="yes"
   ;;
   --disable-stack-protector) stack_protector="no"
@@ -1779,6 +1784,7 @@ disabled with --disable-FEATURE, default is enabled if available
   module-upgrades try to load modules from alternate paths for upgrades
   debug-tcg       TCG debugging (default is disabled)
   debug-info      debugging information
+  lto             Enable Link-Time Optimization.
   sparse          sparse checker
   safe-stack      SafeStack Stack Smash Protection. Depends on
                   clang/llvm >= 3.7 and requires coroutine backend ucontext.
@@ -6965,6 +6971,7 @@ NINJA=$ninja $meson setup \
         -Db_pie=$(if test "$pie" = yes; then echo true; else echo false; fi) \
         ${staticpic:+-Db_staticpic=$staticpic} \
         -Db_coverage=$(if test "$gcov" = yes; then echo true; else echo false; fi) \
+        -Db_lto=$lto \
         -Dmalloc=$malloc -Dmalloc_trim=$malloc_trim -Dsparse=$sparse \
         -Dkvm=$kvm -Dhax=$hax -Dwhpx=$whpx -Dhvf=$hvf \
         -Dxen=$xen -Dxen_pci_passthrough=$xen_pci_passthrough -Dtcg=$tcg \

meson.build

@@ -2080,6 +2080,7 @@ summary_info += {'gprof enabled':     config_host.has_key('CONFIG_GPROF')}
 summary_info += {'sparse enabled':    sparse.found()}
 summary_info += {'strip binaries':    get_option('strip')}
 summary_info += {'profiler':          config_host.has_key('CONFIG_PROFILER')}
+summary_info += {'link-time optimization (LTO)': get_option('b_lto')}
 summary_info += {'static build':      config_host.has_key('CONFIG_STATIC')}
 if targetos == 'darwin'
   summary_info += {'Cocoa support': config_host.has_key('CONFIG_COCOA')}