cmake: detect _GNU_SOURCE not by __GNU_LIBRARY__ only (fallback to _GNU_SOURCE)

Fixes pipe2() on Android-9/armv7-a

Refs: #475

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 1eabc2433fe9ccaf5403c285290390922e5a04e5..a861e7d96340c19e0dcca05eceb0ac4bcec9bded 100644 (file)
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -224,7 +224,14 @@ if (SOLARIS)
 endif()
 
 # Check if _GNU_SOURCE is available.
-CHECK_SYMBOL_EXISTS(__GNU_LIBRARY__ "features.h" _GNU_SOURCE)
+if (NOT _GNU_SOURCE)
+  CHECK_SYMBOL_EXISTS(__GNU_LIBRARY__ "features.h" _GNU_SOURCE)
+
+  if (NOT _GNU_SOURCE)
+    unset(_GNU_SOURCE CACHE)
+    CHECK_SYMBOL_EXISTS(_GNU_SOURCE "features.h" _GNU_SOURCE)
+  endif()
+endif()
 
 if (_GNU_SOURCE)
     add_definitions(-D_GNU_SOURCE)