Partial backport of this commit, to avoid an implicit declaration of
getpid and future build failures:

Author: Noah Oblath <nsoblath@gmail.com>
Date:   Mon Jan 24 15:33:56 2022 -0800

    Nsoblath/issue 163 (#171)

diff -ur apbs-3.0.0.orig/contrib/iapbs/src/apbs_driver.c apbs-3.0.0/contrib/iapbs/src/apbs_driver.c
--- apbs-3.0.0.orig/contrib/iapbs/src/apbs_driver.c	2020-05-30 03:47:30.000000000 +0200
+++ apbs-3.0.0/contrib/iapbs/src/apbs_driver.c	2023-02-06 07:58:46.215007012 +0100
@@ -18,6 +18,11 @@
 
 #include "apbs_driver.h"
 
+#ifdef WIN32
+#include <process.h>
+#else
+#include <unistd.h>
+#endif
 
 /*! \def MAX_BUF_SIZE
     \brief Buffer size for internal APBS string input.