[PATCH] Test -flto on both compiler and linker

[Sergei Trofimovich <slyich@gmail.com>]

Before the change ./configure incorrectly enabled -flto on toolchains
that support -flto on compiler side but don't support -flto on linker
side. This caused incorrect type size detection on nixpkgs' Darwin:

 configure:54594: checking size of size_t
 configure:54600: clang -std=gnu11 -o conftest -g -O2 -flto   conftest.c  >&5
 ld: warning: ignoring file /private/tmp/nix-build-guile-3.0.8.drv-0/conftest-00e93d.o,
   building for macOS-x86_64 but attempting to link with file built
   for unknown-unsupported file format ( 0xDE 0xC0 0x17 0x0B 0x00 0x00 0x00 0x00 0x14 0x00 0x00 0x00 0x80 0x1A 0x00 0x00 )
 Undefined symbols for architecture x86_64:
   "_main", referenced from:
      implicit entry/start for main executable
 ld: symbol(s) not found for architecture x86_64
 clang-11: error: linker command failed with exit code 1 (use -v to see invocation)

Taken from https://github.com/NixOS/nixpkgs/pull/160051#issuecomment-1046105041

The change makes sure -flto support tests basic support of just for
object file generation but for linker as well.

* configure.ac: use AC_LINK_IFELSE instead of AC_COMPILE_IFELSE.
---
 configure.ac | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/configure.ac b/configure.ac
index 827e1c09d..4064533b3 100644
--- a/configure.ac
+++ b/configure.ac
@@ -69,7 +69,7 @@ AC_MSG_CHECKING([whether the compiler supports -flto])
 old_CFLAGS="$CFLAGS"
 LTO_CFLAGS="-flto"
 CFLAGS="$CFLAGS $LTO_CFLAGS"
-AC_COMPILE_IFELSE([AC_LANG_SOURCE([int foo;])],, [LTO_CFLAGS=])
+AC_LINK_IFELSE([AC_LANG_PROGRAM([int foo;], [])],, [LTO_CFLAGS=])
 CFLAGS="$old_CFLAGS"
 if test -n "$LTO_CFLAGS"; then
   AC_MSG_RESULT([yes])
-- 
2.35.1