Update CMake, gcc and clang minimal versions

New requirements
- CMake 3.10
- gcc 8
- clang 8

CMakeLists.txt

-cmake_minimum_required (VERSION 3.4)
+cmake_minimum_required (VERSION 3.10)
 
 # CMake utils
 list(APPEND CMAKE_MODULE_PATH "${CMAKE_CURRENT_SOURCE_DIR}/cmake")
@@ -81,8 +81,8 @@ endif()
 #------------------------------------------------------
 
 # Checks if compiler version is compatible with Pugs sources
-set(GNU_CXX_MIN_VERSION "7.0.0")
-set(CLANG_CXX_MIN_VERSION "5.0.0")
+set(GNU_CXX_MIN_VERSION "8.0.0")
+set(CLANG_CXX_MIN_VERSION "8.0.0")
 
 # Pugs default compiler flags
 set(PUGS_CXX_FLAGS "${PUGS_CXX_FLAGS} -Wall -Wextra -pedantic")
@@ -181,15 +181,7 @@ endif()
 
 #------------------------------------------------------
 # C++ 17 flags
-if(${CMAKE_VERSION} VERSION_LESS "3.8.0")
-  message(WARNING "Please consider to switch to CMake >= 3.8")
-  set(PUGS_CXX_FLAGS "${PUGS_CXX_FLAGS}  -std=gnu++1z")
-  if (CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
-    set(PUGS_CXX_FLAGS "${PUGS_CXX_FLAGS}  -Wno-c++17-extensions")
-  endif()
-else()
 set(CMAKE_CXX_STANDARD "17")
-endif()
 
 #------------------------------------------------------
 # Kokkos configuration