allow installation in nonstandard directories

[GitMensch]

copied from https://raw.githubusercontent.com/lz4/lz4/dev/lib/Makefile, tested - works

[stevengj]

You introduced the uppercase variables, but didn't actually use them in the install target below

[GitMensch]

@stevengj You are right. I see the reasoning of introducing the uppercase variables, but it worked as-is (on SLES).
Do you prefer to drop the "BSD variant" completely or change the Makefile to use the upper-case variables? I'll commit the appropriate change.

[nalimilan]

Better have only the lowercase variants, things get really messy with two variables for each directory.

[stevengj]

Doesn't e.g. make prefix=/foo/bar already work prior to this patch? Setting a variable on the command line should already override that variable in the file even without using ?=.

?= only allows the variable to be overridden by an environment variable, but you can also do that by just running make -e.

[GitMensch]

Doesn't e.g. make prefix=/foo/bar already work prior to this patch?

No, make prefix=/foo/bar install was actually what I've did - and got an access denied for /usr/something. After using the Makefile including in the PR the install worked.

[nalimilan]

I'm surprised to hear it doesn't work, as I use make install DESTDIR=%{buildroot} prefix=%{_prefix} includedir=%{_includedir} libdir=%{_libdir} in the Fedora package and it works fine.

[stevengj]

and got an access denied for /usr/something

An "access denied" error has nothing to do with the Makefile. You just forgot to use sudo.

[GitMensch]

An "access denied" error has nothing to do with the Makefile. You just forgot to use sudo.

No, this has something to do with the Makefile as it was raised when doing

make prefix=/foo/bar install - I did not intended to install anything into system space.

With the changed Makefile the same command worked perfectly fine...

[stevengj]

@GitMensch, what version of make are you using? (What is make -v)

[GitMensch]

@stevengj

GNU Make 4.2.1
Built for i686-pc-linux-gnu

To finish this PR:
Do you prefer to drop the "BSD variant" (upper-case variables) completely or change the Makefile to use the new defined upper-case variables (that will be set from the upper-case variables when passed to make, otherwise from the lower-case variables when passed to make, otherwise from the old default)?
I'll commit the appropriate change.

[stevengj]

I'm still trying to figure out what problem is being solved here. The current Makefile already has a prefix variable. make prefix=foo should already override this in GNU make. The main difference with ?=, as I understand it, arises if e.g. you include our Makefile in another makefile and you want the former's definition to take precedence, but that isn't the case here.