Specify which VM model to enable via an = option (#117)

Instead of employing `-DARGO_VM_???` options for VM management
(and have complicated code to check that exactly one of them is enabled).

Besides simplifying the code, this will enable testing all options more
easily on GitHub actions.

For the moment, the same default has been kept, but it will need to be
revised when #110 is appropriately fixed.

Author: kostis

src/CMakeLists.txt

@@ -26,36 +26,21 @@ foreach(src ${synchronization_sources})
 endforeach(src)
 
 
-# exactly one of these must be enabled. below is some code to ensure this.
-option(ARGO_VM_SHM
-	"Handle virtual addresses using POSIX shared memory. Size-limited." ON)
-option(ARGO_VM_ANONYMOUS
-	"Handle virtual addresses using anonymously-mapped memory. Slow." OFF)
-option(ARGO_VM_MEMFD
-	"Handle virtual addresses using an anonymous memory file. Requires kernel 3.17+." OFF)
-
-set(ARGO_VM_COUNT 0 CACHE INTERNAL "Check for multiple virtual address handlers")
 unset(vm_libs)
-
-if(ARGO_VM_SHM)
+set(ARGO_VM "SHM" CACHE STRING "Select how to handle virtual addresses")
+# Exactly one of these must be enabled (or use the default)
+set_property(CACHE ARGO_VM PROPERTY STRINGS ANONYMOUS MEMFD SHM)
+if(ARGO_VM STREQUAL "MEMFD")
+# "Handle virtual addresses using an anonymous memory file. Requires kernel 3.17+."
+	set(vm_sources memfd.cpp)
+elseif(ARGO_VM STREQUAL "ANONYMOUS")
+# "Handle virtual addresses using anonymously-mapped memory. Slow."
+	set(vm_sources anonymous.cpp)
+else()  # default #if(ARGO_VM STREQUAL "SHM")
+# "Handle virtual addresses using POSIX shared memory. Size-limited."
 	set(vm_sources shm.cpp)
 	set(vm_libs rt)
-	math(EXPR ARGO_VM_COUNT "${ARGO_VM_COUNT} + 1")
-endif(ARGO_VM_SHM)
-
-if(ARGO_VM_ANONYMOUS)
-	set(vm_sources anonymous.cpp)
-	math(EXPR ARGO_VM_COUNT "${ARGO_VM_COUNT} + 1")
-endif(ARGO_VM_ANONYMOUS)
-
-if(ARGO_VM_MEMFD)
-	set(vm_sources memfd.cpp)
-	math(EXPR ARGO_VM_COUNT "${ARGO_VM_COUNT} + 1")
-endif(ARGO_VM_MEMFD)
-
-if(NOT ARGO_VM_COUNT EQUAL 1)
-	message(FATAL_ERROR "${ARGO_VM_COUNT} virtual address handlers selected. Please select exactly one virtual address handler.")
-endif(NOT ARGO_VM_COUNT EQUAL 1)
+endif()
 
 foreach(src ${vm_sources})
 	list(APPEND argo_sources virtual_memory/${src})