[PATCH v3] build: support z/OS (OS/390).

From: Haritha D <harithamma.d@ibm.com>

Since the z/OS linker does not support searching dynamic libraries, and the current setting of CC_LD_DYNPATH results in a directory to be supplied to the link step with no option as the suffix, it causes a linker error because the z/OS LD linker does not accept directories as input. Therefore, -L option is added. Also introduced z/OS (OS/390) as a platform in config.mak.uname

Signed-off-by: Haritha D <harithamma.d@ibm.com>
---
    This PR enables a successful git build on z/OS.
    
    Since the z/OS linker does not support searching dynamic libraries, and
    the current setting of CC_LD_DYNPATH results in a directory to be
    supplied to the link step with no option as the suffix, it causes a
    linker error because the z/OS LD linker does not accept directories as
    input. Therefore, we workaround this by adding the -L option. And,
    Introduced z/OS (OS/390) as a platform in config.mak.uname
    
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Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-1663%2FHarithaIBM%2Fzos-v3
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-1663/HarithaIBM/zos-v3
Pull-Request: https://github.com/git/git/pull/1663

Range-diff vs v2:

 1:  53e211d7a65 ! 1:  2f1ad41bc14 build: support z/OS (OS/390).
     @@ config.mak.uname: ifeq ($(uname_S),NONSTOP_KERNEL)
      +        NO_MEMMEM = YesPlease
      +        NO_GECOS_IN_PWENT = YesPlease
      +        HAVE_STRINGS_H = YesPlease
     -+        NEEDS_MODE_TRANSLATION = YesPlease
     ++       NEEDS_MODE_TRANSLATION = YesPlease
      +endif
       ifeq ($(uname_S),MINGW)
       	ifeq ($(shell expr "$(uname_R)" : '1\.'),2)
       		$(error "Building with MSys is no longer supported")
     -
     - ## configure.ac ##
     -@@ configure.ac: else
     -             CC_LD_DYNPATH=-Wl,+b,
     -           else
     -              CC_LD_DYNPATH=
     -+	     if test "$(uname -s)" = "OS/390"; then
     -+		     CC_LD_DYNPATH=-L
     -+	     fi
     -              AC_MSG_WARN([linker does not support runtime path to dynamic libraries])
     -           fi
     -       fi
 2:  05df5d7e2d5 < -:  ----------- an improvement: removed configure.ac changes

 config.mak.uname | 12 ++++++++++++
 1 file changed, 12 insertions(+)