Increase cmake minimum version

[karczex]

Increase cmake minimum version to cmake >= 3.18

Rationale

Since 3.18 Cmake supports REQUIRED parameter in find_program(), which would simplify cmake files in many places all over codebase.

instead of:

find_program(variable_name NAMES program_name)
if( NOT variable_name)
     message(FATAL_ERROR "program not found")
endif()

may be just:

find_program(variable_name NAMES program_name REQUIRED)

[KFilipek]

Hm, can we set "property" in easy way or rewrite it?

# Configure the ccache as compiler launcher
find_program(CCACHE_FOUND ccache)
if(USE_CCACHE AND CCACHE_FOUND)
	set_property(GLOBAL PROPERTY RULE_LAUNCH_COMPILE ccache)
endif()