Compile issue with PCRE

[Michael Grubb]

There seems to be an error in the configure.ac when using PCRE, more  
specifically related to the regcomp function.

In configure.ac it first checks pcre and if found sets the USE_PCRE  
preprocessor and shell variable.  It
also is using the pcreposix.h and pcreposix library.

Later when doing function tests regcomp is tested for in the C library.

Since PCRE provides a regcomp library but the OS I'm compiling on  
(AIX) does not, it is not seeing the regcomp function, thus  
HAVE_REGCOMP is not getting
defined properly.  Ideally there would be a check before calling  
CHECK_FUNCS([... regcomp ...]) in configure.ac as to whether USE_PCRE  
is true or not.