Increase cmake minimum to 3.6.0.

Remove the unneeded cmake version tests.
Switch to the cmake mechanism for specifying the C++11 standard
3.6.0 is the first version that gets the C++11 flags correct for the Intel compiler

Addresses QMCPACK#832

CMakeLists.txt

@@ -2,15 +2,15 @@ PROJECT(qmcpack)
 
 
 #####################################################
-CMAKE_MINIMUM_REQUIRED(VERSION 2.8.10)
+CMAKE_MINIMUM_REQUIRED(VERSION 3.6.0)
 IF(COMMAND cmake_policy)
   cmake_policy(SET CMP0003 NEW)
-  IF(CMAKE_VERSION VERSION_GREATER 3.1.3 AND NOT $ENV{CRAYPE_VERSION} MATCHES ".")
+  IF(NOT $ENV{CRAYPE_VERSION} MATCHES ".")
     cmake_policy(SET CMP0056 NEW)
     # This policy insures that the CMAKE_EXE_LINKER_FLAGS of the calling project
     # are used in try_compile test cmake projects.
     # CHECK_CXX_SOURCE_COMPILES and others depend on try_compile
-  endif(CMAKE_VERSION VERSION_GREATER 3.1.3 AND NOT $ENV{CRAYPE_VERSION} MATCHES ".")
+  endif(NOT $ENV{CRAYPE_VERSION} MATCHES ".")
   IF(CMAKE_GENERATOR MATCHES "Ninja")
     cmake_policy(SET CMP0058 NEW)
   ENDIF(CMAKE_GENERATOR MATCHES "Ninja")
@@ -374,33 +374,9 @@ ELSE(CMAKE_TOOLCHAIN_FILE)
     MESSAGE(WARNING "No default file for compiler (${COMPILER})")
   ENDIF()
 
-  # Check for the compiler C++11 flag, if we go to requiring >= cmake 3.1.3 we could drop this
-  # and just write
-  # set(CMAKE_CXX_STANDARD_REQUIRED 11 CACHE INTEGER "C++ stangard must be at least 11")
-  SET(CXX11_FLAG "-std=c++11")
-  #check if the CXX compiler supports -std=c++11 option
-  include(CheckCXXCompilerFlag) # works with clean load paths and non system compilers with CMP0056
-  CHECK_CXX_COMPILER_FLAG(${CXX11_FLAG} CXX_COMPILER_SUPPORT_CXX11)
-
-  # Force the flag on Cray with Intel compiler, because the Cray wrapper
-  #  prints an warning that interferes with the flag detection code
-  #  with older versions of CMake.
-  IF($ENV{CRAYPE_VERSION} MATCHES ".")
-    IF( ${COMPILER} MATCHES "Intel" AND NOT CXX_COMPILER_SUPPORT_CXX11)
-      SET(CXX_COMPILER_SUPPORT_CXX11 TRUE)
-      MESSAGE(STATUS "Forcing C++11 support on Cray with Intel")
-    ENDIF()
-  ENDIF()
-
-  IF (CXX_COMPILER_SUPPORT_CXX11)
-    # avoid repeated -std=c++11 flag
-    STRING(REPLACE "++" "\\+\\+" CXX11_FLAG_MATCH ${CXX11_FLAG})
-    IF(NOT CMAKE_CXX_FLAGS MATCHES ${CXX11_FLAG_MATCH})
-      SET (CMAKE_CXX_FLAGS "${CXX11_FLAG} ${CMAKE_CXX_FLAGS}")
-    ENDIF()
-  ELSE()
-    MESSAGE(FATAL_ERROR "A compiler supporting C++11 is required. Use a newer C++ compiler.")
-  ENDIF()
+  set(CMAKE_CXX_STANDARD 11)
+  set(CMAKE_CXX_STANDARD_REQUIRED ON)
+  set(CMAKE_CXX_EXTENSIONS OFF)
 
   #-------------------------------------------------------------------
   #  check MPI installation

src/CMakeLists.txt

@@ -327,11 +327,6 @@ ENDIF()
   #ENDIF(BUILD_QMCTOOLS)
 
   if (BUILD_UNIT_TESTS) #{
-    SET(HAS_TARGET_COMPILE_DEFINITIONS 1)
-    IF (CMAKE_VERSION VERSION_LESS "2.8.11")
-      MESSAGE("CMake version 2.8.10 - some unit tests will not be built")
-      SET(HAS_TARGET_COMPILE_DEFINITIONS 0)
-    ENDIF()
     #Unit test directories
     INCLUDE_DIRECTORIES(${PROJECT_SOURCE_DIR}/external_codes/catch)
     INCLUDE(${PROJECT_SOURCE_DIR}/CMake/unit_test.cmake)
@@ -349,12 +344,10 @@ ENDIF()
     SUBDIRS(QMCHamiltonians/tests)
     SUBDIRS(type_traits/tests)
 
-    IF (HAS_TARGET_COMPILE_DEFINITIONS)
-      SUBDIRS(ParticleBase/tests)
-      SUBDIRS(Estimators/tests)
-      SUBDIRS(QMCDrivers/tests)
-      SUBDIRS(QMCApp/tests)
-    ENDIF()
+    SUBDIRS(ParticleBase/tests)
+    SUBDIRS(Estimators/tests)
+    SUBDIRS(QMCDrivers/tests)
+    SUBDIRS(QMCApp/tests)
   endif() #}
 
 endif() #}}}

src/QMCDrivers/CMakeLists.txt

@@ -95,10 +95,8 @@ ENDIF(QMC_BUILD_LEVEL GREATER 1)
 # create libqmc 
 ####################################
 ADD_LIBRARY(qmcdriver ${QMCDRIVERS})
-IF (HAS_TARGET_COMPILE_DEFINITIONS)
-  ADD_LIBRARY(qmcdriver_unit ${QMCDRIVERS})
-  USE_FAKE_RNG(qmcdriver_unit)
-ENDIF()
+ADD_LIBRARY(qmcdriver_unit ${QMCDRIVERS})
+USE_FAKE_RNG(qmcdriver_unit)
 #IF(QMC_BUILD_STATIC)
 #  ADD_LIBRARY(qmcdriver STATIC ${QMCDRIVERS})
 #ELSE(QMC_BUILD_STATIC)