cmake: Pass -DMIN_LOG_LEVEL as compiler definition

- Check if MIN_LOG_LEVEL value is a number 0-3, default to
  INFO (1) or ignore it in Release mode
- When TRAVIS_CI_BUILD is ON the default is DEBUG (0)
- Add local.mk.example

src/nvim/CMakeLists.txt

@@ -108,6 +108,10 @@ if(NOT MSVC)
     ${CONV_SOURCES} PROPERTIES COMPILE_FLAGS "${COMPILE_FLAGS} -Wno-conversion")
 endif()
 
+if(DEFINED MIN_LOG_LEVEL)
+  add_definitions(-DMIN_LOG_LEVEL=${MIN_LOG_LEVEL})
+endif()
+
 get_directory_property(gen_cdefs COMPILE_DEFINITIONS)
 foreach(gen_cdef ${gen_cdefs} DO_NOT_DEFINE_EMPTY_ATTRIBUTES)
   if(NOT "${gen_cdef}" MATCHES "INCLUDE_GENERATED_DECLARATIONS")