Make ld executable configurable

Tools like this should be configurable in a cross or native file. In
particular, if we are cross-compiling (with an executable wrapper like
qemu-arm), the build system ld is not necessarily able to manipulate
host system objects.

Signed-off-by: Simon McVittie <smcv@collabora.com>

gio/tests/meson.build

@@ -586,8 +586,9 @@ if not meson.is_cross_build() or meson.has_exe_wrapper()
   # (https://reviews.llvm.org/D58234). FIXME: This test could be enabled for
   # LLVM once that support is in a stable release.
   objcopy = find_program('objcopy', required : false)
+  ld = find_program('ld', required : false)
 
-  if build_machine.system() == 'linux' and cc.get_id() == 'gcc' and objcopy.found()
+  if build_machine.system() == 'linux' and cc.get_id() == 'gcc' and objcopy.found() and ld.found()
     test_gresource_binary = custom_target('test5.gresource',
       input : 'test5.gresource.xml',
       output : 'test5.gresource',
@@ -616,7 +617,7 @@ if not meson.is_cross_build() or meson.has_exe_wrapper()
     test_resources_binary = custom_target('test_resources.o',
       input : test_gresource_binary,
       output : 'test_resources.o',
-      command : ['ld',
+      command : [ld,
                  '-r',
                  '-b','binary',
                  '@INPUT@',