DOC: update few references to the linux* targets and change them to linux-glibc

The INSTALL guide, the Lua doc and the Prometheus exporter's README all
used to reference "linux2628", "linux26" or even "linux". These were all
updated to consistently reflect "linux-glibc" instead. The default options
were updated there as well so that it should build cleanly on most distros.
diff --git a/contrib/prometheus-exporter/README b/contrib/prometheus-exporter/README
index d02c64d..1d1ec0a 100644
--- a/contrib/prometheus-exporter/README
+++ b/contrib/prometheus-exporter/README
@@ -14,7 +14,7 @@
 component for everyone want to use it. So you need to explicity build HAProxy
 with the PROMEX service, using the Makefile variable "EXTRA_OBJS". For instance:
 
-    > make TARGET=linux2628 EXTRA_OBJS="contrib/prometheus-exporter/service-prometheus.o"
+    > make TARGET=linux-glibc EXTRA_OBJS="contrib/prometheus-exporter/service-prometheus.o"
 
 if HAProxy provides the PROMEX service, the following build option will be
 reported by the command "haproxy -vv":