move setting of default CMAKE_INSTALL_{BIN,INCLUDE,LIB}DIR before first use

[h-vetinari]

One of the things I had misdiagnosed in #876 was how I was getting lib64 and other such things into the build, when nothing should have been setting them like that.

Now after rebasing the patches to 0.2.0, I realize that this is simply due to the fact that the default values (in case the various CMAKE_INSTALL_* quantities are not explicitly overriden) are defined after their first use further up in CMakeLists.txt:

sentencepiece/CMakeLists.txt

Lines 60 to 66 in 17d7580

	if (UNIX)
	include(GNUInstallDirs)
	set(prefix ${CMAKE_INSTALL_PREFIX})
	set(exec_prefix "\${prefix}")
	set(libdir "\${exec_prefix}/${CMAKE_INSTALL_LIBDIR}")
	set(includedir "\${prefix}/${CMAKE_INSTALL_INCLUDEDIR}")
	else()

Fix this by moving the setting of the default values up before they're first used.

Also rename CMAKE_INSTALL_INCDIR -> CMAKE_INSTALL_INCLUDEDIR, which corresponds to the GNUInstallDirs default that CMake itself documents.