[PATCH] Makefile: check that tcl/tk is installed

By default running `make install` in the root directory of the
project will set TCLTK_PATH to `wish` and then go into the "git-gui"
and "gitk-git" sub-directories to build and install these 2
sub-projects.

When Tcl/Tk is not installed, the above will succeed if gettext
is installed, as Tcl/Tk is only required as a substitute for msgfmt
when msgfmt is not installed. But then running the installed gitk
and git-gui will fail.

If neither Tcl/Tk nor gettext are installed, then processing po
files will fail in the git-gui directory. The error message when
this happens is very confusing to new comers as it is difficult
to understand that we tried to use Tcl/Tk as a substitute for
msgfmt, and that the solution is to either install gettext or
Tcl/Tk, or to set both NO_GETTEXT and NO_TCLTK.

To improve the current behavior when Tcl/Tk is not installed,
let's just check that TCLTK_PATH points to something and error
out right away if this is not the case.

This should benefit people who actually want to install and use
git-gui or gitk as they will have to install Tcl/Tk anyway, and
it is better for them if they are told about installing it soon
in the build process, rather than if they have to debug it after
installing.

For people who don't want to use git-gui or gitk, this forces
them to specify NO_TCLTK. If they don't have gettext, this will
make it much easier to fix the problems they would have had to
fix anyway. If they have gettext, setting NO_TCLTK is a small
additional step they will have to make, but it might be a good
thing as it will not install what they don't want and it will
build a bit faster.

Signed-off-by: Christian Couder <redacted>
---
 Makefile | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/Makefile b/Makefile
index ee9d5eb11e..ada6164e15 100644
--- a/Makefile
+++ b/Makefile

@@ -1636,6 +1636,13 @@ ifeq ($(TCLTK_PATH),)
 NO_TCLTK = NoThanks
 endif
 
+ifndef NO_TCLTK
+	has_tcltk := $(shell type $(TCLTK_PATH) 2>/dev/null)
+	ifndef has_tcltk
+$(error "Tcl/Tk is not installed ('$(TCLTK_PATH)' not found). Consider setting NO_TCLTK or installing Tcl/Tk")
+	endif
+endif
+
 ifeq ($(PERL_PATH),)
 NO_PERL = NoThanks
 endif

-- 
2.15.0.165.g42c5887