build: fix building with LTO

Building with link-time optimization requires some tricks explained
in [1].

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=48200#c28

m4/compiler_options.m4

@@ -150,9 +150,9 @@ fi
 AC_DEFUN([NM_LTO],
 [AC_ARG_ENABLE(lto, AS_HELP_STRING([--enable-lto], [Enable Link Time Optimization for smaller size (default: no)]))
 if (test "${enable_lto}" = "yes"); then
-	CC_CHECK_FLAG_APPEND([lto_flags], [CFLAGS], [-flto])
+	CC_CHECK_FLAG_APPEND([lto_flags], [CFLAGS], [-flto -flto-partition=none])
 	if (test -n "${lto_flags}"); then
-		CFLAGS="-flto $CFLAGS"
+		CFLAGS="-flto -flto-partition=none $CFLAGS"
 	else
 		AC_MSG_ERROR([Link Time Optimization -flto is not supported.])
 	fi

meson.build

@@ -147,6 +147,15 @@ if enable_ld_gc
   common_ldflags += test_ldflags
 endif
 
+enable_lto = get_option('b_lto')
+if enable_lto
+  # meson already adds '-flto'
+  lto_flag = '-flto-partition=none'
+  assert(cc.has_argument(lto_flag), '-flto-partition=none not supported. Disable link-time optimization with -Db_lto=false.')
+  common_flags += lto_flag
+  common_ldflags += lto_flag
+endif
+
 if nm_debug
   common_flags += cc.get_supported_arguments([
     '-fno-strict-aliasing',
@@ -986,7 +995,7 @@ if enable_valgrind
 endif
 output += '\n'
 output += '  code coverage: ' + get_option('b_coverage').to_string() + '\n'
-output += '  LTO: ' + get_option('b_lto').to_string() + '\n'
+output += '  LTO: ' + enable_lto.to_string() + '\n'
 output += '  Linker garbage collection: ' + enable_ld_gc.to_string() + '\n'
 output += '  JSON validation for libnm: ' + enable_json_validation.to_string() + '\n'
 output += '  crypto: ' + crypto + ' (have-gnutls: ' + crypto_gnutls_dep.found().to_string() + ', have-nss: ' + crypto_nss_dep.found().to_string() + ')\n'

shared/nm-utils/nm-macros-internal.h

@@ -1245,7 +1245,7 @@ fcn_name (lookup_type val) \
 
 #define _NM_BACKPORT_SYMBOL_IMPL(version, return_type, orig_func, versioned_func, args_typed, args) \
 return_type versioned_func args_typed; \
-return_type versioned_func args_typed \
+__attribute__ ((externally_visible)) return_type versioned_func args_typed \
 { \
     return orig_func args; \
 } \