perf: add SLANG_INC for slang.h

CFLAGS was previously hard coded to contain "-I/usr/include/slang" to
work with hosts that have "/usr/include/slang/slang.h" as well as hosts
that have "/usr/include/slang.h". This path can cause compile warnings
like:

cc1: warning: '/usr/include/slang' doesn't exists.

or

cc1: warning: include location "/usr/include/slang" is unsafe for
cross-compilation [-Wpoison-system-directories]

Then in some cases warnings become errors if WERROR is enabled hence
build errors.

To fix this issue, we can use -idirafter to downgrade the priority of the
default hard coded path. We can also make the slang include directory
a variable, to allow the user to specify SLANG_INC and set their own
include location. And add a '=' prefix to indicate better
compatibility with sysroot/cross compile cases.

Signed-off-by: Liang Li <liang.li@windriver.com>
Signed-off-by: Tom Zanussi <tom.zanussi@linux.intel.com>