Conditionals should enclose complete statements like variables or rules definitions. Automake cannot deal with conditionals used inside a variable definition, for instance, and is not even able to diagnose this situation. The following example would not work:
# This syntax is not understood by Automake
AM_CPPFLAGS = \
-DFEATURE_A \
if WANT_DEBUG
-DDEBUG \
endif
-DFEATURE_B
However the intended definition of AM_CPPFLAGS can be achieved
with
if WANT_DEBUG
DEBUGFLAGS = -DDEBUG
endif
AM_CPPFLAGS = -DFEATURE_A $(DEBUGFLAGS) -DFEATURE_B
or
AM_CPPFLAGS = -DFEATURE_A
if WANT_DEBUG
AM_CPPFLAGS += -DDEBUG
endif
AM_CPPFLAGS += -DFEATURE_B
More details and examples of conditionals are described alongside various Automake features in this manual (see Conditional Subdirectories, see Conditional Sources, see Conditional Programs, see Conditional Libtool Libraries, see Conditional Libtool Sources).