Building pkgconfig-0.25 on OS X 10.6 using --with-installed-popt with popt-1.14 (long-existing version build from debian's source tarball, supplied via fink):

gcc -DHAVE_CONFIG_H -I. -DPKG_CONFIG_PC_PATH="\"/sw/lib/pkgconfig:/sw/share/pkgconfig:/usr/X11/lib/pkgconfig:/usr/X11R6/lib/pkgconfig:/usr/lib/pkgconfig\"" -I./glib-1.2.10 -I./glib-1.2.10   -I/sw/include  -Os -MT parse.o -MD -MP -MF .deps/parse.Tpo -c -o parse.o parse.c
parse.c: In function 'parse_libs':
parse.c:759: warning: passing argument 3 of 'poptParseArgvString' from incompatible pointer type
parse.c: In function 'parse_libs_private':
parse.c:808: warning: passing argument 3 of 'poptParseArgvString' from incompatible pointer type
parse.c: In function 'parse_cflags':
parse.c:849: warning: passing argument 3 of 'poptParseArgvString' from incompatible pointer type

Same with popt-1.16 (recently-released from debian). If I do not use --with-installed-popt, those parse.c warnings disappear. The prototype in popt.h is slightly different in popt-1.16 (and comparable in 1.14):

int poptParseArgvString(
  const char * s,
  int * argcPtr,
  const char *** argvPtr
);

vs one included with pkg-config:

int poptParseArgvString(
  char * s,
  int * argcPtr,
  char *** argvPtr
);

and parse.c is coded to match the included one (not 'const').