ENH: set version as shell variable

[jbrockmendel]

The first step in the instructions is to edit the make file manually. Instead of editing the TAG and setting github username manually there, we should have the user export TAG as a shell variable. (github username we can get with git config --get user.name under mild assumptions).

Two upsides:

1. don't ask the user to modify any files inplace
2. On line 39 of the readme there is an ambiguous make pandas/dist/<>.tar.gz that can be made unambiguous using shell expansion.

[WillAyd]

Makes sense. Could also pass directly from CLI . Either would be an upgrade

https://stackoverflow.com/questions/2826029/passing-additional-variables-from-command-line-to-make

[TomAugspurger]

FWIW, you can do this now.

diff --git a/Makefile b/Makefile
index 0d79fa0..bdfe2b8 100644
--- a/Makefile
+++ b/Makefile
@@ -8,6 +8,10 @@ TARGZ=pandas-$(PANDAS_VERSION).tar.gz
 # to ensure pushd and popd works
 SHELL := /bin/bash
 
+
+print-tag:
+	echo $(TAG)
+
 # -----------------------------------------------------------------------------
 # Host filesystem initialization
 # -----------------------------------------------------------------------------

bash-5.0$ make print-tag TAG=v1.0.0
echo v1.0.0
v1.0.0
bash-5.0$ TAG=v1.0.0 make print-tag
echo v1.0.0
v1.0.0

but you need to remember to always use it.