feat(cmake): add conditional builds for falcoctl and rules

There are systems, like Nix derivations where the build process
does not have network access in order to enforce reproducibility.
This patch allows people building Falco to optionally skip the build
of falcoctl with `-DADD_FALCOCTL_DEPENDENCY=OFF` and point to their own
self-backed, or pre-fetched rules files with
`-DFALCOSECURITY_RULES_FALCO_PATH=<some-path>` and
`-DFALCOSECURITY_RULES_LOCAL_PATH=<some-path>`.

For context, I needed to apply these patches while building the
project with Nix in https://github.com/tembleking/falco-nix but I think
that would be benefitial for the community to have also these options
open, and that would also make Falco feasible to be added to the
nixpkgs repository at https://github.com/nixos/nixpkgs

Signed-off-by: Fede Barcelona <fede_rico_94@hotmail.com>

cmake/modules/rules.cmake

@@ -15,6 +15,7 @@
 include(GNUInstallDirs)
 include(ExternalProject)
 
+if(NOT DEFINED FALCOSECURITY_RULES_FALCO_PATH)
 # falco_rules.yaml
 set(FALCOSECURITY_RULES_FALCO_VERSION "falco-rules-3.1.0")
 set(FALCOSECURITY_RULES_FALCO_CHECKSUM "SHA256=3b617920c0b66128627613e591a954eb9572747a4c287bc13b53b38786250162")
@@ -28,10 +29,13 @@ ExternalProject_Add(
   INSTALL_COMMAND ""
   TEST_COMMAND ""
 )
+endif()
 
+if(NOT DEFINED FALCOSECURITY_RULES_LOCAL_PATH)
 # falco_rules.local.yaml
 set(FALCOSECURITY_RULES_LOCAL_PATH "${PROJECT_BINARY_DIR}/falcosecurity-rules-local-prefix/falco_rules.local.yaml")
 file(WRITE "${FALCOSECURITY_RULES_LOCAL_PATH}" "# Your custom rules!\n")
+endif()
 
 if(NOT DEFINED FALCO_ETC_DIR)
 	set(FALCO_ETC_DIR "${CMAKE_INSTALL_FULL_SYSCONFDIR}/falco")