Use pkg-config for abseil-cpp detection if available

This should make things a bit easier.

meson.build

@@ -40,6 +40,25 @@ os_deps = []
 have_posix = false
 have_win = false
 
+# Let's use pkg-config if available. This will also fallback to the subproject
+# if pkg-config is not found, instead of CMake or manual library detection.
+# This might be surprising, but is hopefully a temporary state until recent
+# abseil versions become the norm.
+absl_base_dep = dependency('absl_base', required : false)
+absl_flags_dep = dependency('absl_flags', required : false)
+absl_strings_dep = dependency('absl_strings', required : false)
+absl_synchronization_dep = dependency('absl_synchronization', required : false)
+
+# If we have the base dep, assume the rest should be present to
+if absl_base_dep.found()
+  absl_dep = [
+    absl_base_dep,
+    absl_flags_dep,
+    absl_strings_dep,
+    absl_synchronization_dep
+  ]
+else
+  warning('Could not find abseil-cpp with pkg-config, trying CMake-based library detection.')
   absl_dep = dependency('absl', method : 'cmake',
     modules : [
       'absl::base',
@@ -86,6 +105,7 @@ if not absl_dep.found()
       absl_dep += cpp.find_library(l, required : false)
     endforeach
   endif
+endif
 
 if ['darwin', 'ios'].contains(host_system)
   os_cflags = ['-DWEBRTC_MAC']