cmake if

example

message(STATUS "env costmap_2d_INCLUDE_DIR " $ENV{costmap_2d_INCLUDE_DIR})
if (x"$ENV{costmap_2d_INCLUDE_DIR}" STREQUAL x"")
  find_package(costmap_2d REQUIRED)
  message(STATUS "costmap_2d_INCLUDE_DIRS " ${costmap_2d_INCLUDE_DIRS})
  set(costmap_2d_INCLUDE_DIR ${costmap_2d_INCLUDE_DIRS})
else()
  message(STATUS "get costmap_2d_INCLUDE_DIR from env")
  set(costmap_2d_INCLUDE_DIR $ENV{costmap_2d_INCLUDE_DIR})
endif()
message(STATUS "costmap_2d_INCLUDE_DIR " ${costmap_2d_INCLUDE_DIR})

STREQUAL example 2 !!

if("${CV_SIMD128}" STREQUAL "")
  add_definitions(-DCV_SIMD128=1)
else()
  add_definitions(-DCV_SIMD128=${CV_SIMD128})
endif()

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https://yuiwong.org/gitlab/documentation/compiler