Convert configuration file parsing to use flex

Previously, repmgr was using a very simple ad-hoc string-based parser,
which had various limitations and allowed configuration files to be
created in a way which could cause confusion and/or unexpected
behaviour.

For example, it accepted strings enclosed in single quotes, but treated
strings enclosed in double quotes literally. A node_name defined thusly:

    node_name="somenode"

would result in the literal value '"somenode"' being used, which could
lead to unobvious errors along the lines of:

    no record found for ""somenode""

The configuration file parser has been adapted from the one used by
PostgreSQL itself, so behaves more-or-less identically (though some
functions such as file inclusion are not supported in repmgr).

This makes configuration parsing more robust and consistent;
additionally, error reporting will be more precise.

Note this does mean that some repmgr.conf items previously accepted
as valid by repmgr will now be rejected; in particular this includes
strings containing spaces which are not enclosed in single quotes.

Makefile.global.in

@@ -2,6 +2,7 @@
 # Makefile.global.in
 # @configure_input@
 
+
 # Can only be built using pgxs
 USE_PGXS=1
 
@@ -29,3 +30,11 @@ include $(PGXS)
 REPMGR_VERSION=$(shell awk '/^\#define REPMGR_VERSION / { print $3; }' ${repmgr_abs_srcdir}/repmgr_version.h.in | cut -d '"' -f 2)
 REPMGR_RELEASE_DATE=$(shell awk '/^\#define REPMGR_RELEASE_DATE / { print $3; }' ${repmgr_abs_srcdir}/repmgr_version.h.in | cut -d '"' -f 2)
 
+FLEX = flex
+
+##########################################################################
+#
+# Global targets and rules
+
+%.c: %.l
+	$(FLEX) $(FLEXFLAGS) -o'$@' $<