try this...

CMakeLists.txt

 cmake_minimum_required (VERSION 3.9)
 
+# prevent in-source builds and give warning message
+if ("${CMAKE_BINARY_DIR}" STREQUAL "${CMAKE_SOURCE_DIR}") 
+  message (FATAL_ERROR "In-source builds are disabled.
+    Please create a subfolder and use `cmake ..` inside it.
+    NOTE: cmake will now create CMakeCache.txt and CMakeFiles/*.
+          You must delete them, or cmake will refuse to work.")
+endif () 
+
 project (
   corsika
   VERSION 8.0.0
@@ -14,7 +22,7 @@ enable_language (Fortran)
 set (CMAKE_INSTALL_MESSAGE LAZY)
 
 # directory for local cmake modules
-set(CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/CMakeModules)
+set (CMAKE_MODULE_PATH ${PROJECT_SOURCE_DIR}/CMakeModules)
 include (CorsikaUtilities) # a few cmake function
 
 set (CMAKE_CXX_STANDARD 17)
@@ -22,50 +30,66 @@ set (CMAKE_CXX_EXTENSIONS OFF)
 enable_testing ()
 set (CTEST_OUTPUT_ON_FAILURE 1)
 
-option(COVERAGE "Generate coverage data" OFF)
-if(COVERAGE)
-  set(CMAKE_BUILD_TYPE "Debug")
-endif()
+# add custom target to switch to "debug" build with "make debug"
+ADD_CUSTOM_TARGET(debug
+  COMMAND ${CMAKE_COMMAND} -DCMAKE_BUILD_TYPE=Debug ${CMAKE_SOURCE_DIR}
+  COMMAND ${CMAKE_COMMAND} --build ${CMAKE_BINARY_DIR} --target all
+  COMMENT "Switch CMAKE_BUILD_TYPE to Debug"
+  )
 
-# Set a default build type if none was specified
-set(default_build_type "Release")
-if(EXISTS "${CMAKE_SOURCE_DIR}/.git")
-  set(default_build_type "Debug")
-endif()
+# add custom target to switch to "release" build with "make release"
+ADD_CUSTOM_TARGET(release
+  COMMAND ${CMAKE_COMMAND} -DCMAKE_BUILD_TYPE=Release ${CMAKE_SOURCE_DIR}
+  COMMAND ${CMAKE_COMMAND} --build ${CMAKE_BINARY_DIR} --target all
+  COMMENT "Switch CMAKE_BUILD_TYPE to Release"
+  )
 
-if(NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
-  message(STATUS "Setting build type to '${default_build_type}' as no other was specified.")
-  set(CMAKE_BUILD_TYPE "${default_build_type}" CACHE
+# add custom target to switch to "coverage" build with "make coverage"
+ADD_CUSTOM_TARGET(coverage
+  COMMAND ${CMAKE_COMMAND} -DCMAKE_BUILD_TYPE=Coverage ${CMAKE_SOURCE_DIR}
+  COMMAND ${CMAKE_COMMAND} --build ${CMAKE_BINARY_DIR} --target all
+  COMMENT "Switch CMAKE_BUILD_TYPE to Coverage"
+  )
+
+# Set a default build type if none was specified
+set (default_build_type "Release")
+if (EXISTS "${CMAKE_SOURCE_DIR}/.git")
+  set (default_build_type "Debug")
+endif ()
+
+if (NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
+  message (STATUS "Setting build type to '${default_build_type}' as no other was specified.")
+  set (CMAKE_BUILD_TYPE "${default_build_type}" CACHE
       STRING "Choose the type of build." FORCE)
   # Set the possible values of build type for cmake-gui
-  set_property(CACHE CMAKE_BUILD_TYPE PROPERTY STRINGS
+  set_property (CACHE CMAKE_BUILD_TYPE PROPERTY STRINGS
     "Debug" "Release" "MinSizeRel" "RelWithDebInfo")
-endif(NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
-
+else (NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
+  message (STATUS "Build type is: ${CMAKE_BUILD_TYPE}")
+endif (NOT CMAKE_BUILD_TYPE AND NOT CMAKE_CONFIGURATION_TYPES)
 
 # enable warnings and disallow non-standard language
-set(CMAKE_CXX_FLAGS "-Wall -pedantic -Wextra -Wno-ignored-qualifiers")
-set(CMAKE_CXX_FLAGS_DEBUG "-O0 -g")
-set(CMAKE_CXX_FLAGS_RELEASE "-O3 -g") # -O2 would not trade speed for size, neither O2/3 use fast-math
-set(CMAKE_Fortran_FLAGS "-std=legacy")
+set (CMAKE_CXX_FLAGS "-Wall -pedantic -Wextra -Wno-ignored-qualifiers")
+set (CMAKE_CXX_FLAGS_DEBUG "-O0 -g")
+set (CMAKE_CXX_FLAGS_RELEASE "-O3 -g") # -O2 would not trade speed for size, neither O2/3 use fast-math
+set (CMAKE_Fortran_FLAGS "-std=legacy")
 
 # setup coverage target
-set(CMAKE_CXX_FLAGS_COVERAGE "${CMAKE_CXX_FLAGS_DEBUG} --coverage")
-set(CMAKE_EXE_LINKER_FLAGS "${CMAKE_EXE_LINKER_FLAGS_DEBUG} --coverage")
+set (CMAKE_CXX_FLAGS_COVERAGE "-O0 -g --coverage")
+set (CMAKE_EXE_LINKER_FLAGS_COVERAGE "--coverage")
 
 # clang produces a lot of unecessary warnings without this:
-add_compile_options("$<$<OR:$<CXX_COMPILER_ID:Clang>,$<CXX_COMPILER_ID:AppleClang>>:-Wno-nonportable-include-path>")
+add_compile_options ("$<$<OR:$<CXX_COMPILER_ID:Clang>,$<CXX_COMPILER_ID:AppleClang>>:-Wno-nonportable-include-path>")
 
 # COAST - interface
 if (WITH_COAST)
-  message(STATUS "Compiling CORSIKA8 for the use with COAST/corsika7.")
-  add_compile_options("-fPIC")
-endif()
+  message (STATUS "Compiling CORSIKA8 for the use with COAST/corsika7.")
+  add_compile_options ("-fPIC")
+endif ()
 
 #add_custom_target (corsika_pre_build)
 #add_custom_command (TARGET corsika_pre_build PRE_BUILD COMMAND "${PROJECT_SOURCE_DIR}/pre_compile.py")
 
-
 find_package (Pythia8) # optional
   
 find_package (Eigen3 REQUIRED)