[Freeipa-devel] [freeipa PR#718][comment] configure: fix AC_CHECK_LIB usage

[lslebodn]

  URL: https://github.com/freeipa/freeipa/pull/718
Title: #718: configure: fix AC_CHECK_LIB usage

lslebodn commented:
"""
>It does not explicitly say what constitutes an unspecified value though.

```, , ``` or ``` , [] ,``` are considered as unspecified values and therefore default action was used.

BTW it is not peculiar behaviour because most project define custom LIBS using the 3rd argument
and not after invocation of macro.
e.g.
```
AC_CHECK_HEADER(krad.h, [], [AC_MSG_ERROR([krad.h not found])])
-AC_CHECK_LIB(krad, main, [], [AC_MSG_ERROR([libkrad not found])])
+AC_CHECK_LIB(krad, main, [KRAD_LIBS="-lkrad"], [AC_MSG_ERROR([libkrad not found])])
-KRAD_LIBS="-lkrad"
```

And link to online documentation https://www.gnu.org/software/autoconf/manual/autoconf.html#Libraries
"""

See the full comment at https://github.com/freeipa/freeipa/pull/718#issuecomment-294844095