[Cmake] EXEC_PROGRAM and it's output variable

[Terry Lorber]

FYI: If EXEC_PROGRAM and CONFIGURE_FILE are in the same location, it works.

tgl
----------------------------------
PROJECT(Config)

EXEC_PROGRAM(date
   ARGS +%Y%m%d
   OUTPUT_VARIABLE BUILD_DATE
)

CONFIGURE_FILE(${Config_SOURCE_DIR}/builddate.h.in
   ${Config_BINARY_DIR}/builddate.h
)