Gnu make conditionals in Makefile.am

[Ralf Reißing <reissing at informatik dot uni-stuttgart dot de>]

I am trying to do the following: 
Depending on the target platform (available in $target from
AC_CANONICAL_SYSTEM in configure.in) a variable OS_FLAGS in the
Makefile should be set to specifix values,
e.g. on Linux:  OS_FLAGS = -DUNIX -DLINUX

I tried to add gnumake conditionals for this to Makefile.am, like

ifneq (,$findstring(linux,$(target)))
	OS_FLAGS = -DUNIX -DLINUX
endif

Unfortunately, automake thinks these conditionals are automake
conditionals ("endif without if") and does not process the file.
Is there a way of "masking" conditionals from automake so it
simply passes the code on to Makefile.in?

Or is there an alternative way of getting the desired conditional
setting of OS_FLAGS done in configure.in?

TIA Ralf