From fd4fe7cbc5559b0928661dab6d178921b50a15de Mon Sep 17 00:00:00 2001
From: Christoph Hertzberg <chtz@informatik.uni-bremen.de>
Date: Tue, 24 Jul 2018 22:56:15 +0200
Subject: [PATCH] Fixed issue which made documentation not getting built
 anymore

---
 Eigen/src/Core/util/Macros.h | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/Eigen/src/Core/util/Macros.h b/Eigen/src/Core/util/Macros.h
index 73a9883b9..b15819f7d 100644
--- a/Eigen/src/Core/util/Macros.h
+++ b/Eigen/src/Core/util/Macros.h
@@ -518,6 +518,8 @@
 #endif
 
 // Does the compiler support C99?
+// Need to include <cmath> to make sure _GLIBCXX_USE_C99 gets defined
+#include <cmath>
 #ifndef EIGEN_HAS_C99_MATH
 #if EIGEN_MAX_CPP_VER>=11 && \
     ((defined(__STDC_VERSION__) && (__STDC_VERSION__ >= 199901))       \