This is a CMake Environment Variable. Its initial value is taken from the calling process environment.

Default compilation flags to be used when compiling a specific dialect of an assembly language. ASM<DIALECT>FLAGS can be ASMFLAGS, ASM_NASMFLAGS, ASM_MASMFLAGS or ASM-ATTFLAGS. Will only be used by CMake on the first configuration to determine ASM<DIALECT> default compilation flags, after which the value for ASM<DIALECT>FLAGS is stored in the cache as CMAKE_ASM<DIALECT>_FLAGS. For any configuration run (including the first), the environment variable will be ignored if the CMAKE_ASM<DIALECT>_FLAGS variable is defined.