BUILD: makefile: rename "linux2628" to "linux-glibc" and remove older targets

The linux targets have become more than confusing over time. We used to
have "linux2628" to match the features available in kernels 2.6.28 and
above, without consideration for the libc, and due to many new features
appearing later in kernels, some other options were added that are not
enabled by default in linux2628, so this target doesn't make any sense
anymore. The older ones (linux 2.2, linux 2.4, ...) do not make sense
either since these versions are not supported anymore. Let's clean things
up by creating a new "linux-glibc" target that matches what is available
by default on Linux kernels and glibc present on supported distros at the
time of release. Other libc implementation may use a custom or generic
target or be added later if needed.

All the older linux targets were removed.
diff --git a/Makefile b/Makefile
index 4785b6d..1e1057a 100644
--- a/Makefile
+++ b/Makefile
@@ -5,7 +5,7 @@
 #
 # When in doubt, invoke help, possibly with a known target :
 #   [g]make help
-#   [g]make help TARGET=linux
+#   [g]make help TARGET=linux-glibc
 #
 # By default the detailed commands are hidden for a cleaner output, but you may
 # see them by appending "V=1" to the make command.
@@ -141,8 +141,8 @@
 #### TARGET system
 # Use TARGET=<target_name> to optimize for a specifc target OS among the
 # following list (use the default "generic" if uncertain) :
-#    generic, linux22, linux24, linux24e, linux26, solaris,
-#    freebsd, openbsd, netbsd, cygwin, haiku, custom, aix51, aix52
+#    linux-glibc, solaris, freebsd, openbsd, netbsd, cygwin,
+#    haiku, aix51, aix52, osx, generic, custom
 TARGET =
 
 #### TARGET CPU
@@ -318,34 +318,8 @@
   set_target_defaults = $(call default_opts,USE_POLL USE_TPROXY)
 endif
 
-# Linux 2.2
-ifeq ($(TARGET),linux22)
-  set_target_defaults = $(call default_opts, \
-    USE_POLL USE_TPROXY USE_LIBCRYPT USE_DL USE_RT)
-endif
-
-# Standard Linux 2.4 with netfilter but without epoll()
-ifeq ($(TARGET),linux24)
-  set_target_defaults = $(call default_opts, \
-    USE_POLL USE_TPROXY USE_LIBCRYPT USE_DL USE_RT USE_CRYPT_H USE_NETFILTER)
-endif
-
-# Enhanced Linux 2.4 with netfilter and epoll() patch > 0.21
-ifeq ($(TARGET),linux24e)
-  set_target_defaults = $(call default_opts, \
-    USE_POLL USE_TPROXY USE_LIBCRYPT USE_DL USE_RT USE_CRYPT_H USE_NETFILTER  \
-    USE_EPOLL USE_MY_EPOLL)
-endif
-
-# Standard Linux 2.6 with netfilter and standard epoll()
-ifeq ($(TARGET),linux26)
-  set_target_defaults = $(call default_opts, \
-    USE_POLL USE_TPROXY USE_LIBCRYPT USE_DL USE_RT USE_CRYPT_H USE_NETFILTER  \
-    USE_EPOLL USE_FUTEX USE_PRCTL)
-endif
-
-# Standard Linux >= 2.6.28 with netfilter, epoll, tproxy and splice
-ifeq ($(TARGET),linux2628)
+# For linux >= 2.6.28 and glibc
+ifeq ($(TARGET),linux-glibc)
   set_target_defaults = $(call default_opts, \
     USE_POLL USE_TPROXY USE_LIBCRYPT USE_DL USE_RT USE_CRYPT_H USE_NETFILTER  \
     USE_CPU_AFFINITY USE_THREAD USE_EPOLL USE_FUTEX USE_LINUX_TPROXY          \
@@ -758,9 +732,8 @@
 	@echo
 	@echo "Please choose the target among the following supported list :"
 	@echo
-	@echo "   linux2628, linux26, linux24, linux24e, linux22, solaris,"
-	@echo "   freebsd, netbsd, osx, openbsd, aix51, aix52, cygwin, haiku,"
-	@echo "   generic, custom"
+	@echo "   linux-glibc, solaris, freebsd, openbsd, netbsd, cygwin,"
+	@echo "   haiku, aix51, aix52, osx, generic, custom"
 	@echo
 	@echo "Use \"generic\" if you don't want any optimization, \"custom\" if you"
 	@echo "want to precisely tweak every option, or choose the target which"
@@ -824,8 +797,8 @@
 	     echo;\
 	   else \
 	     echo "TARGET not set, you may pass 'TARGET=xxx' to set one among :";\
-	     echo "  linux2628, linux26, linux24, linux24e, linux22, solaris, freebsd,"; \
-	     echo "  netbsd, osx, openbsd, aix51, aix52, cygwin, haiku, generic, custom"; \
+	     echo "  linux-glibc, solaris, freebsd, netbsd, osx, openbsd,"; \
+	     echo "  aix51, aix52, cygwin, haiku, generic, custom"; \
 	   fi
 	$(Q)echo;echo "Enabled features for TARGET '$(TARGET)' (disable with 'USE_xxx=') :"
 	$(Q)set -- $(foreach opt,$(patsubst USE_%,%,$(use_opts)),$(if $(USE_$(opt)),$(opt),)); echo "  $$*" | (fmt || cat) 2>/dev/null