Add descriptions of invocation switches and environment variables.

Add a new variable $TCADDINCL which allows addition of header files
for the testfunc scripts. It can be used, e.g., for testing a certain
typedef in a special include file, by using:
  env TCADDINCL='#include <special.h>' ./helpers/TestCompile sizeof blah_t
where the typedef of blah_t is assumed to be in <special.h>


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/1.3.x@85576 13f79535-47bb-0310-9956-ffa450edef68