AC_CHECK_PROG is incompatible with alias

[styx]

There is a conflict with make behavior and  AC_CHECK_PROG macro behavior
(maybe) when prog. is aliased (OS: cygwin)  :

for example:
$ type -a hhc
hhc is aliased to `/cygdrive/c/Util/Dev/htmlhelp/hhc.exe'


configure.ac:
AC_CHECK_PROG([FOUND],[hhc],[yes],[no])
if test  "x$FOUND" = "xno"; then
    AC_MSG_ERROR(['hhc' is required !])
else
    AC_SUBST(HHC,hhc)
fi

configure:
checking for hhc... (cached) yes

Makefile.am :
$(CHM_FILENAME) : $(SUB_DIR)/index.hhp $(SUB_DIR)/index.hhc
$(SUB_DIR)/index.hhk
 cd $(SUB_DIR) && $(HHC) index.hhp

make:
cd tdg/en/html && hhc index.hhp
/bin/sh: line 1: hhc: command not found


Why does AC_CHECK_PROG  find hhc ?