slang.h is in sysroot/usr/include, no need to use -I

Signed-off-by: Alex Suykov <alex.suykov@gmail.com>

--- a/Makefile.in
+++ b/Makefile.in
@@ -5,7 +5,7 @@
 CPP = @CPP@
 CFLAGS = @CFLAGS@
 LDFLAGS = @LDFLAGS@
-CPPFLAGS = -D_GNU_SOURCE -I/usr/include/slang @CPPFLAGS@ 
+CPPFLAGS = -D_GNU_SOURCE @CPPFLAGS@ 
 GNU_LD = @GNU_LD@
 
 VERSION = @VERSION@