NetWare build tweaks.

mwccnlm: search directory of referencing file first for #includes.
(backport r1370592, 1370615)
Make use of CFLAGS from environment.
Added default maxerrors value 1 so that the compiler breaks
after 1st error; can be overwritten with CC_MAX_ERRORS var.
(backport 1370616)


git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/branches/2.4.x@1371164 13f79535-47bb-0310-9956-ffa450edef68