Add better default flags

CMakeLists.txt

@@ -1,7 +1,11 @@
 cmake_minimum_required(VERSION 3.5 FATAL_ERROR)
 
+set(CMAKE_USER_MAKE_RULES_OVERRIDE ${CMAKE_SOURCE_DIR}/cmake/UserOverride.cmake)
+
 project(lfortran C CXX)
 
+set(CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/cmake)
+
 if (NOT CMAKE_BUILD_TYPE)
     set(CMAKE_BUILD_TYPE Release
         CACHE STRING "Build type (Debug, Release)" FORCE)

cmake/UserOverride.cmake

@@ -0,0 +1,26 @@
+# This overrides the default CMake Debug and Release compiler options.
+# The user can still specify different options by setting the
+# CMAKE_CXX_FLAGS_[RELEASE,DEBUG] variables (on the command line or in the
+# CMakeList.txt). This files serves as better CMake defaults and should only be
+# modified if the default values are to be changed. Project specific compiler
+# flags should be set in the CMakeList.txt by setting the CMAKE_CXX_FLAGS_*
+# variables.
+
+if (CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+    # g++
+    set(common "-Wall -Wextra -Wno-unused-parameter -fno-common")
+    set(CMAKE_CXX_FLAGS_RELEASE_INIT "${common} -O3 -march=native -funroll-loops -DNDEBUG")
+    set(CMAKE_CXX_FLAGS_DEBUG_INIT   "${common} -g -ggdb")
+elseif (CMAKE_CXX_COMPILER_ID STREQUAL "Intel")
+    # icpc
+    set(common "-Wall -fno-common")
+    set(CMAKE_CXX_FLAGS_RELEASE_INIT "${common} -xHOST -O3")
+    set(CMAKE_CXX_FLAGS_DEBUG_INIT   "${common} -g -O0")
+elseif (CMAKE_CXX_COMPILER_ID MATCHES Clang)
+    # clang
+    set(common "-Wall -Wextra -Wno-unused-parameter")
+    set(CMAKE_CXX_FLAGS_RELEASE_INIT "${common} -O3 -march=native -funroll-loops -DNDEBUG")
+    set(CMAKE_CXX_FLAGS_DEBUG_INIT   "${common} -g -ggdb")
+elseif (CMAKE_CXX_COMPILER_ID STREQUAL "PGI")
+    # pgcpp
+endif ()