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/doc/lua.txt b/doc/lua.txt
index cbc7f3d..a0a1d61 100644
--- a/doc/lua.txt
+++ b/doc/lua.txt
@@ -597,8 +597,7 @@
    cd haproxy-1.6.2
 
  - build HAProxy:
-   make TARGET=linux \
-        USE_DL=1 \
+   make TARGET=linux-glibc \
         USE_LUA=1 \
         LUA_LIB=/opt/lua-5.3.1/lib \
         LUA_INC=/opt/lua-5.3.1/include