Conditionals should enclose complete statements, such as variable or rule definitions.
For instance, Automake cannot deal with conditionals used inside a variable definition, 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
The intended definition of
AM_CPPFLAGS can be achieved
if WANT_DEBUG DEBUGFLAGS = -DDEBUG endif AM_CPPFLAGS = -DFEATURE_A $(DEBUGFLAGS) -DFEATURE_B
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, Conditional Sources, Conditional Programs, Conditional Libtool Libraries, and Conditional Libtool Sources).