Should not write file to CMAKE_INSTALL_PREFIX when configuration

[reyoung]

Paddle/cmake/inference_lib.cmake

Lines 176 to 185 in d54ad9f

	set(version_file ${CMAKE_INSTALL_PREFIX}/version.txt)
	file(WRITE ${version_file}
	"GIT COMMIT ID: ${PADDLE_GIT_COMMIT}\n"
	"WITH_MKL: ${WITH_MKL}\n"
	"WITH_GPU: ${WITH_GPU}\n")
	if(WITH_GPU)
	file(APPEND ${version_file}
	"CUDA version: ${CUDA_VERSION}\n"
	"CUDNN version: v${CUDNN_MAJOR_VERSION}\n")
	endif()

This line will write files to CMAKE_INSTALL_PREFIX, which is not the good way to install file. Use install command of cmake.

This lines will make Paddle failed to compile when the user has no write permission to CMAKE_INSTALL_PREFIX.

[luotao1]

Discussed with @reyoung, CMAKE_INSTALL_PREFIX will be changed to FLUID_INSTALL_DIR, and the default path of FLUID_INSTALL_DIR is ${CMAKE_BINARY_DIR}/fluid_install_dir.