Define `export` rule for `CApp` shared makefile

With GNU `make` unlike with `amu`, it is considered an error to make a target
for which no rule is defined. Therefore we need an explicit no-op `export`
rule in the cross-compiling `CApp` makefile.

GNUmakefiles/CApp

@@ -274,4 +274,10 @@ include $(wildcard *.dd)
 
 endif
 
+export${CUSTOMEXP}: export_${PHASE}
+	@${NOP}
+
+export_ export_hdrs export_libs:
+	@${NOP}
+
 # EOF