CMakeLists: Fix ppoll() feature detection.

Use check_cxx_symbol_exists() instead of check_symbol_exists()
for the subtle reason explained in the comment added in this
patch.

Change-Id: Id949b8f2c7af39eeef3a848d3bdbc7639432f38a

CMakeLists.txt

@@ -40,8 +40,16 @@ else()
 	if(NOT NINJA_FORCE_PSELECT)
 		# Check whether ppoll() is usable on the target platform.
 		# Set -DUSE_PPOLL=1 if this is the case.
-		include(CheckSymbolExists)
-		check_symbol_exists(ppoll poll.h HAVE_PPOLL)
+		#
+		# NOTE: Use check_cxx_symbol_exists() instead of check_symbol_exists()
+		# because on Linux, <poll.h> only exposes the symbol when _GNU_SOURCE
+		# is defined.
+		#
+		# Both g++ and clang++ define the symbol by default, because the C++
+		# standard library headers require it, but *not* gcc and clang, which
+		# are used by check_symbol_exists().
+		include(CheckCXXSymbolExists)
+		check_cxx_symbol_exists(ppoll poll.h HAVE_PPOLL)
 		if(HAVE_PPOLL)
 			add_compile_definitions(USE_PPOLL=1)
 		endif()