Make make-install-prefix a common option again.

I had made some build-system related options not automatically apply to
modules with customized build systems (e.g. Qt, CMake, "custom"). But
this seems like the kind of option we'd want to have in common, as noted
in a bug report to me by Andre Wöbbeking, so make make-install-prefix
apply to all build systems by default if it's a global option.

This can, of course, still be overridden for individual modules or
module-sets.

doc/index.docbook

@@ -2157,7 +2157,7 @@ generated by the script.
 
 <row id="conf-make-install-prefix">
 <entry>make-install-prefix</entry>
-<entry>Module setting overrides global (build system option)</entry>
+<entry>Module setting overrides global</entry>
 <entry>Set this variable to a space-separated list, which is interpreted as a
 command and its options to precede the <userinput><command>make</command> <option>install</option></userinput> command used to install
 modules. This is useful for installing packages with &sudo; for example, but

modules/ksb/BuildSystem.pm

@@ -38,7 +38,7 @@ sub _maskGlobalBuildSystemOptions
     my $ctx = $module->buildContext();
     my @buildSystemOptions = qw(
         cmake-options configure-flags custom-build-command cxxflags
-        make-install-prefix make-options run-tests use-clean-install
+        make-options run-tests use-clean-install
     );
 
     for my $opt (@buildSystemOptions) {