[LyX/master] Cmake build: Amend 41e409f8: Use std::call_once only if it is actually available

[Kornel Benko]

commit 2d49342ce39a2f6177ce8f460d454932f4c90bc2
Author: Kornel Benko <kor...@lyx.org>
Date:   Sun Aug 7 11:32:53 2016 +0200

    Cmake build: Amend 41e409f8: Use std::call_once only if it is actually 
available
---
 development/cmake/ConfigureChecks.cmake |   11 +++++++++++
 development/cmake/config.h.cmake        |    3 +++
 2 files changed, 14 insertions(+), 0 deletions(-)

diff --git a/development/cmake/ConfigureChecks.cmake 
b/development/cmake/ConfigureChecks.cmake
index 146330a..61190c8 100644
--- a/development/cmake/ConfigureChecks.cmake
+++ b/development/cmake/ConfigureChecks.cmake
@@ -184,6 +184,17 @@ check_cxx_source_compiles(
   "
 HAVE_DEF_MAKE_UNIQUE)
 
+check_cxx_source_compiles(
+  "
+  #include <mutex>
+  static std::once_flag flag;
+  int main() {
+    std::call_once(flag, [](){ return; });
+    return(0);
+  }
+  "
+LYX_USE_STD_CALL_ONCE)
+
 set(USE_LLVM_LIBCPP)
 set(STD_STRING_USES_COW)
 set(USE_GLIBCXX_CXX11_ABI)
diff --git a/development/cmake/config.h.cmake b/development/cmake/config.h.cmake
index 95148b6..dbd2c83 100644
--- a/development/cmake/config.h.cmake
+++ b/development/cmake/config.h.cmake
@@ -91,6 +91,9 @@ ${Include_used_spellchecker}
 
 #cmakedefine HAVE_DEF_MAKE_UNIQUE 1
 
+// Define to 1 if std::call_once is supported by the compiler
+#cmakedefine LYX_USE_STD_CALL_ONCE 1
+
 #endif // config.h guard