pgsql: Add configure infrastructure (--with-llvm) to enable LLVMsuppor

Add configure infrastructure (--with-llvm) to enable LLVM support.

LLVM will be used for *optional* Just-in-time compilation
support. This commit just adds the configure infrastructure that
detects LLVM.

No documentation has been added for the --with-llvm flag, that'll be
added after the actual supporting code has been added.

Author: Andres Freund
Discussion: https://postgr.es/m/20170901064131.tazjxwus3k2w3ybh@alap3.anarazel.de

Branch
------
master

Details
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https://git.postgresql.org/pg/commitdiff/5b2526c83832e4e8a9f8db0389904ed2fb50ed37

Modified Files
--------------
aclocal.m4                    |   1 +
config/llvm.m4                | 108 ++++++
configure                     | 788 ++++++++++++++++++++++++++++++++++++++----
configure.in                  |  62 +++-
src/Makefile.global.in        |  11 +
src/include/pg_config.h.in    |  19 +
src/include/pg_config.h.win32 |  19 +
7 files changed, 939 insertions(+), 69 deletions(-)