config.mak.uname: set NO_REGEX=NeedsStartEnd on AIX

Set the NO_REGEX=NeedsStartEnd Makefile flag by default on AIX.

Since commit 2f8952250a ("regex: add regexec_buf() that can work on a
non NUL-terminated string", 2016-09-21) git has errored out at
compile-time if the regular expression library doesn't support
REG_STARTEND.

While looking through Google search results for the use of NO_REGEX I
found a Chef recipe that set this on AIX[1], looking through the
documentation for the latest version of AIX (7.2, released October
2015) shows that its regexec() doesn't have REG_STARTEND.

1. https://github.com/chef/omnibus-software/commit/e247e36761#diff-3df898345d670979b74acc0bf71d8c47
2. https://www.ibm.com/support/knowledgecenter/ssw_aix_72/com.ibm.aix.basetrf2/regexec.htm

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.mak.uname

@@ -237,6 +237,7 @@ ifeq ($(uname_S),AIX)
 	NO_MKDTEMP = YesPlease
 	NO_STRLCPY = YesPlease
 	NO_NSEC = YesPlease
+	NO_REGEX = NeedsStartEnd
 	FREAD_READS_DIRECTORIES = UnfortunatelyYes
 	INTERNAL_QSORT = UnfortunatelyYes
 	NEEDS_LIBICONV = YesPlease