Makefile: remove the NO_R_TO_GCC_LINKER flag

Change our default CC_LD_DYNPATH invocation to something GCC likes
these days. Since the GCC 4.6 release unknown flags haven't been
passed through to ld(1). Thus our previous default of CC_LD_DYNPATH=-R
would cause an error on modern GCC unless NO_R_TO_GCC_LINKER was set.

This CC_LD_DYNPATH flag is really obscure, and I don't expect anyone
except those working on git development ever use this.

It's not needed to simply link to libraries like say libpcre,
but *only* for those cases where we're linking to such a library not
present in the OS's library directories. See e.g. ldconfig(8) on Linux
for more details.

I use this to compile my git with a LIBPCREDIR=$HOME/g/pcre2/inst as
I'm building that from source, but someone maintaining an OS package
is almost certainly not going to use this. They're just going to set
USE_LIBPCRE=YesPlease after installing the libpcre dependency,
which'll point to OS libraries which ld(1) will find without the help
of CC_LD_DYNPATH.

Another thing that helps mitigate any potential breakage is that we
detect the right type of invocation in configure.ac, which e.g. HP/UX
uses[1], as does IBM's AIX package[2]. From what I can tell both AIX
and Solaris packagers are building git with GCC, so I'm not adding a
corresponding config.mak.uname default to cater to their OS-native
linkers.

Now for an overview of past development in this area:

Our use of "-R" dates back to 455a7f3275 ("More portability.",
2005-09-30). Soon after that in bbfc63dd78 ("gcc does not necessarily
pass runtime libpath with -R", 2006-12-27) the NO_R_TO_GCC flag was
added, allowing optional use of "-Wl,-rpath=".

Then in f5b904db6b ("Makefile: Allow CC_LD_DYNPATH to be overriden",
2008-08-16) the ability to override this flag to something else
entirely was added, as some linkers use neither "-Wl,-rpath," nor
"-R".

From what I can tell we should, with the benefit of hindsight, have
made this change back in 2006. GCC & ld supported this type of
invocation back then, or since at least binutils-gdb.git's[3]
a1ad915dc4 ("[...]Add support for -rpath[...]", 1994-07-20).

Further reading and prior art can be found at [4][5][6][7]. Making a
plain "-R" an error seems from reading those reports to have been
introduced in GCC 4.6 released on March 25, 2011[8], but I couldn't
confirm this with absolute certainty, its release notes are ambiguous
on the subject, and I couldn't be bothered to try to build & bisect it
against GCC 4.5.

1. https://public-inbox.org/git/20190516093412.14795-1-avarab@gmail.com/
2. https://www.ibm.com/developerworks/aix/library/aix-toolbox/alpha.html
3. git://sourceware.org/git/binutils-gdb.git
4. https://github.com/tsuna/boost.m4/issues/15
5. https://bugzilla.gnome.org/show_bug.cgi?id=641416
6. https://stackoverflow.com/questions/12629042/g-4-6-real-error-unrecognized-option-r
7. https://curl.haxx.se/mail/archive-2014-11/0005.html
8. https://gcc.gnu.org/gcc-4.6/changes.html

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

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