ini: Fix parsing of variable with spaces before the =

At least from what the source code suggests, a variable declaration in
the config file as this should be supported:
DFLAGS = -I/usr/include/d

But in reality, if you live spaces like that, the variable value will
get uppercased.

This commit fixes this problem to properly parse the extra spaces.
man: Mention variable names are converted to uppercase
ini: Add support to define undefined variables

Similarly to Make files, the syntax `VAR ?= value` is added to define
variables only when they were previously undefined.

In this case, if the environment variable `VAR` was undefined, then it
will be defined with the value `value`, otherwise the previous content
of `VAR` will be preserved.

This is useful to define default flags in the config file that can be
easily overridden by using environment variables.
Merge pull request #3076 from leandro-lucarella-sociomantic/cond-conf-vars2

ini: Add support to define undefined variables