gio/tests/{meson.build,pollable.c}: Determine libutil SONAME at build time

Calling `dlopen()` with `libutil.so` makes the installed tests depend on
having glibc's development files installed. To avoid this, we can work
out the runtime library name at build time and `dlopen` that instead.

This approach is [taken from libfprint][1], thanks to Marco Trevisan.

[1]: f401f399a8

gio/tests/meson.build

@@ -12,6 +12,21 @@ test_c_args = [
   '-UG_DISABLE_ASSERT',
 ]
 
+# workaround for https://github.com/mesonbuild/meson/issues/6880
+if build_machine.system() == 'linux'
+  libutil_name = 'libutil'
+  libutil = run_command('sh', '-c',
+    '''ldconfig -p | grep -o "[[:space:]]@0@\.so\(\.[0-9]\+\)\?\b"'''
+    .format(libutil_name)).stdout().strip().split('\n')
+
+  if libutil.length() > 0
+      message('Found libutil as @0@'.format(libutil[0]))
+      test_c_args += '-DLIBUTIL_SONAME="@0@"'.format(libutil[0])
+  else
+      warning('libutil not found')
+  endif # libutil.length() > 0
+endif # build_machine.system() == 'linux'
+
 if host_machine.system() == 'windows'
   common_gio_tests_deps += [iphlpapi_dep, winsock2, cc.find_library ('secur32')]
 endif