Create missing "src" directory

Apparently, this is now necessary when compiling in another directory
than the top-level

generated_headers.am

@@ -3,7 +3,7 @@ define extract_interface
 	@grep -h "^ *$1" $^ | sed 's/^ *$1//;' >> $@ || { rm $@; exit 1; }
 endef
 
-elpa test:
+elpa test src:
 	@mkdir $@
 
 test/shared: | test
@@ -33,7 +33,7 @@ elpa/elpa_generated_fortran_interfaces.h: $(wildcard $(top_srcdir)/src/elpa2/ker
 	$(call extract_interface,!f>)
 	$(call extract_interface,#!f>)
 
-src/fortran_constants.X90: $(top_srcdir)/src/fortran_constants.h
+src/fortran_constants.X90: $(top_srcdir)/src/fortran_constants.h | src
 	@$(CPP) $(CPPFLAGS) -I$(top_builddir)/ -I$(top_srcdir)/ -I. $< -o $@_ || { rm -f $@; exit 1; }
 	@awk '/!ELPA_C_DEFINE/ {gsub(/!ELPA_C_DEFINE/, "\n"); gsub(/NEWLINE/, "\n"); print;}' < $@_ > $@ || { rm -f $@; exit 1; }
 	@rm $@_