Right now we have scripts that need to redirect their
output referencing the file written out by a custom
build target to indicate the current build type.  We
might instead be able to pass that information in as
a define using the $<CONFIG> generator expression (or
maybe more than just that one if that's not enough
for MSVC).

CMakeLists.txt:
add_custom_command(
	OUTPUT script_type.txt
	COMMAND ${CMAKE_COMMAND} -DBUILD_TYPE="$<CONFIG>" -P ${CMAKE_SOURCE_DIR}/script.cmake
	)
add_custom_target(script DEPENDS script_type.txt)

script.cmake:
message("BUILD_TYPE: ${BUILD_TYPE}")
file(WRITE script_type.txt "${BUILD_TYPE}")

output:

cmake .. -DCMAKE_BUILD_TYPE=Debug && make script
[100%] Generating script_type.txt
BUILD_TYPE: Debug
[100%] Built target script

cmake .. -DCMAKE_BUILD_TYPE=Release && make script
[100%] Generating script_type.txt
BUILD_TYPE: Release
[100%] Built target script