FIXUP: Add missing pkg_config calls for system libs

Only call pkg_config in case of system libraries.
It may fail otherwise (e.g. mac).

Pick-to: 112-based
Change-Id: I1d506a507eadec7423e71785777f50518f378861
Reviewed-on: https://codereview.qt-project.org/c/qt/qtwebengine-chromium/+/528389
Reviewed-by: Peter Varga <pvarga@inf.u-szeged.hu>
Reviewed-by: Michal Klocek <michal.klocek@qt.io>

chromium/third_party/libevent/BUILD.gn

@@ -84,15 +84,17 @@ shim_headers("libevent_shim") {
   headers = [ "event.h" ]
 }
 
-pkg_config("libevent_from_pkgconfig") {
-  packages = [ "libevent" ]
-}
+if (use_system_libevent) {
+  pkg_config("libevent_from_pkgconfig") {
+    packages = [ "libevent" ]
+  }
 
-source_set("system_libevent") {
-  deps = [
-    ":libevent_shim",
-  ]
-  public_configs = [ ":libevent_from_pkgconfig" ]
+  source_set("system_libevent") {
+    deps = [
+      ":libevent_shim",
+    ]
+    public_configs = [ ":libevent_from_pkgconfig" ]
+  }
 }
 
 group("libevent") {

chromium/third_party/re2/BUILD.gn

@@ -68,10 +68,6 @@ fuzzer_test("third_party_re2_fuzzer") {
   deps = [ ":re2" ]
 }
 
-pkg_config("re2_from_pkgconfig") {
-  packages = [ "re2" ]
-}
-
 shim_headers("re2_shim") {
   root_path = "src/re2"
   prefix = "re2/"
@@ -83,9 +79,15 @@ shim_headers("re2_shim") {
   ]
 }
 
-source_set("system_re2") {
-  deps = [ ":re2_shim" ]
-  public_configs = [ ":re2_from_pkgconfig" ]
+if (use_system_re2) {
+  pkg_config("re2_from_pkgconfig") {
+    packages = [ "re2" ]
+  }
+
+  source_set("system_re2") {
+    deps = [ ":re2_shim" ]
+    public_configs = [ ":re2_from_pkgconfig" ]
+  }
 }
 
 group("re2") {