+@node Compiler Flags, Recursing into Subdirectories, Compiling Libraries, User's Manual
+@section Compiler Flags
+
+OMK follows the same philosophy for flag variables as does Automake. The
+variables with AM_ prefix (e.g. AM_CPPFLAGS) are supposed to be used by
+the package developer and variable without that prefix (e.g. CPPFLAGS)
+are reserved for the user. The following
+
+@defvar AM_CPPFLAGS
+Preprocessor switches.
+@end defvar
+
+@defvar AM_CFLAGS
+C compiler switches.
+@end defvar
+
+
+@defvar INCLUDES
+ Directives passed to the C or C++ compiler with additional directories
+ to be searched for header files. In most cases you need to specify an
+ absolute path. To specify a directory relative to the source
+ directory, you can use the @code{$(SOURCES_DIR)} variable, which
+ refers to the directory, where @file{Makefile.omk} is located. This
+ variable applies to all compilations invoked in the current directory.
+
+ @example
+ INCLUDES = -I$(SOURCES_DIR)/my_include_dir
+ @end example
+@end defvar
+
+@defvar DEFS
+ Directives passed to the C or C++ compiler with preprocessor macro
+ definitions. This variable applies to all compilations invoked in the
+ current directory.
+
+ @example
+ DEFS = -DDEBUG=1
+ @end example
+@end defvar
+
+@c FIXME: INCLUDES variable should not be set by rtlinux rules.
+
+@node Recursing into Subdirectories, Dependency Tracking, Compiler Flags, User's Manual