Minor problem with configure (2.4.4)

[sndive at gmail.com]

configure expands
  AC_DEFINE(_POSIX_C_SOURCE, 200112L, Define to activate features from
IEEE Stds 1003.1-2001)

to

#define _POSIX_C_SOURCE 200112L

that causes problems because _POSIX_C_SOURCE is defined by system
headers and I get hideous
warnings during compilation.

I tried to fix it by adding AC_UNDEFINE(_POSIX_C_SOURCE);
before AC_DEFINE but autoconf 2.59 won't take it:

configure.in:273: error: possibly undefined macro: AC_UNDEFINE
      If this token and others are legitimate, please use
m4_pattern_allow.
      See the Autoconf documentation.

Ideas?