test: add sigabbrev_np() for systems where it's not available

sigabbrev_np() was first added to glibc 2.32 (Aug 2020) which is too recent
for some of the distributions we support.

test/meson.build

@@ -2,6 +2,7 @@ pwtest_sources = [
   'pwtest.h',
   'pwtest-implementation.h',
   'pwtest.c',
+  'pwtest-compat.c',
 ]
 
 pwtest_deps = [
@@ -30,6 +31,13 @@ pwtest_lib = static_library(
   include_directories: pwtest_inc,
 )
 
+test('test pwtest',
+    executable('test-pwtest',
+               'test-pwtest.c',
+               include_directories: pwtest_inc,
+               link_with: pwtest_lib)
+)
+
 # Compilation only, this is the example file for how pwtest works and most
 # of its tests will fail.
 executable('test-example',