Allow Unity build.

Two parts:
- missing include guard in solver_utils.h
- exclude schur_specializations from unity build.  This
  can only be done with cmake >= 3.16, so only allow
  unity build for these versions.

Cuts build time by 5 minutes with default settings.
libceres.a shrinks from 7549968 bytes to 7009696 bytes.

Change-Id: I833ac8266623da0e725cc2f07a6415b4fa72bc18

internal/ceres/CMakeLists.txt

diff --git a/internal/ceres/CMakeLists.txt b/internal/ceres/CMakeLists.txt
index 6dc7262..140b250 100644
--- a/internal/ceres/CMakeLists.txt
+++ b/internal/ceres/CMakeLists.txt
@@ -170,6 +170,21 @@
   file(GLOB CERES_INTERNAL_SCHUR_FILES generated/*_d_d_d.cc)
 endif (SCHUR_SPECIALIZATIONS)
 
+# The generated specializations of the Schur eliminator include
+# schur_eliminator_impl.h which defines EIGEN_CACHEFRIENDLY_PRODUCT_THRESHOLD
+# to a different value than Eigen's default.  Depending on the order of files
+# in the unity build this can lead to clashes.  Additionally, these files are
+# already generated in a way which leads to fairly large compilation units,
+# so the gains from a unity build would be marginal.  Since this property is
+# not available before cmake 3.16, unity build is only available from this
+# version on.
+if (NOT CMAKE_VERSION VERSION_LESS 3.16)
+  set_source_files_properties(${CERES_INTERNAL_SCHUR_FILES} PROPERTIES
+    SKIP_UNITY_BUILD_INCLUSION ON)
+elseif(CMAKE_UNITY_BUILD)
+  message(FATAL_ERROR "Unity build requires cmake 3.16 or newer.  Please unset CMAKE_UNITY_BUILD.")
+endif()
+
 # Build the list of dependencies for Ceres based on the current configuration.
 find_package(Threads QUIET)
 list(APPEND CERES_LIBRARY_PUBLIC_DEPENDENCIES Threads::Threads)

internal/ceres/solver_utils.h

diff --git a/internal/ceres/solver_utils.h b/internal/ceres/solver_utils.h
index 85fbf37..5715cb7 100644
--- a/internal/ceres/solver_utils.h
+++ b/internal/ceres/solver_utils.h
@@ -28,6 +28,9 @@
 //
 // Author: sameeragarwal@google.com (Sameer Agarwal)
 
+#ifndef CERES_INTERNAL_SOLVER_UTILS_H_
+#define CERES_INTERNAL_SOLVER_UTILS_H_
+
 #include <algorithm>
 #include <string>
 
@@ -59,3 +62,5 @@
 
 }  // namespace internal
 }  // namespace ceres
+
+#endif  // CERES_INTERNAL_SOLVER_UTILS_H_