Configure and make improvements.

Configure:

* Use standard make variables, e.g. CFLAGS, rather than our own, e.g. CINCLUDE
* Add PG_CONFIG var for configuring custom pg_config location
* Don't error if xml_config or pg_config is missing (but error if libs/headers not found)
* Check for zlib.h header
* Check for lz4frame.h header when liblz4 is present

Make:

* Use gcc-style auto dependencies
* Put src list at the top since it is most frequently modified
* Add clean-all target to also remove auto-generated config files

test/patch/rhel-package.patch

@@ -1,11 +1,20 @@
 --- pgbackrest.spec
 +++ pgbackrest.spec
-@@ -9,7 +9,7 @@
+@@ -8,8 +8,7 @@
+ Url:		http://www.pgbackrest.org/
  Source0:	https://github.com/pgbackrest/pgbackrest/archive/release/%{version}.tar.gz
  Source1:	pgbackrest-conf.patch
- Patch0:		pgbackrest-libxmlinclude.patch
+-Patch0:		pgbackrest-libxmlinclude.patch
 -BuildRequires:	openssl-devel zlib-devel postgresql%{pgmajorversion}-devel
 +BuildRequires:	openssl-devel zlib-devel postgresql%{pgmajorversion}-devel lz4-devel
  Requires:	postgresql-libs
  
  
+@@ -26,7 +25,6 @@
+ 
+ %prep
+ %setup -q -n %{name}-release-%{version}
+-%patch0 -p0
+ 
+ %build
+ pushd src