Configurable source_base et al. in bootstrap

[Sergey Poznyakoff]

Hello,

Some projects place gnulib sources into a directory named other than
`lib'. Therefore, I propose the following changes (m4_base and others
are added for the sake of completeness):

diff --git a/build-aux/bootstrap b/build-aux/bootstrap
index 6fa1553..87a2798 100755
--- a/build-aux/bootstrap
+++ b/build-aux/bootstrap
@@ -96,6 +96,11 @@ package=`sed -n "$extract_package_name" configure.ac` || exit
 gnulib_name=lib$package
 
 build_aux=build-aux
+source_base=lib
+m4_base=m4
+doc_base=doc
+tests_base=tests
+
 # Extra files from gnulib, which override files from other sources.
 gnulib_extra_files="
        $build_aux/install-sh
@@ -490,11 +495,11 @@ gnulib_tool_options="\
  --import\
  --no-changelog\
  --aux-dir $bt/$build_aux\
- --doc-base $bt/doc\
+ --doc-base $bt/$doc_base\
  --lib $gnulib_name\
- --m4-base $bt/m4/\
- --source-base $bt/lib/\
- --tests-base $bt/tests\
+ --m4-base $bt/$m4_base/\
+ --source-base $bt/$source_base/\
+ --tests-base $bt/$tests_base\
  --local-dir $local_gl_dir\
  $gnulib_tool_option_extras\
 "

Regards,
Sergey