Parameterise builddir

Same as 08086cd, really.

l3build.dtx

@@ -63,11 +63,12 @@
 \luavarset{testfiledir}{maindir .. "/testfiles"}  {Directory containing test files}
 \luavarset{testsuppdir}{testfiledir .. "/support"}{Directory containing test-specific support files}
 \luavarseparator
-\luavarset{distribdir}{maindir .. "/build/distrib"}{Directory for generating distribution structure}
-\luavarset{localdir}  {maindir .. "/build/local"}  {Directory for extracted files in \enquote{sandboxed} \TeX{} runs}
-\luavarset{testdir}   {maindir .. "/build/test"}   {Directory for running tests}
-\luavarset{typesetdir}{maindir .. "/build/doc"}    {Directory for building documentation}
-\luavarset{unpackdir} {maindir .. "/build/unpack"} {Directory for unpacking sources}
+\luavarset{builddir}  {maindir .. "/build"}   {Directory for building and testing}
+\luavarset{distribdir}{builddir .. "/distrib"}{Directory for generating distribution structure}
+\luavarset{localdir}  {builddir .. "/local"}  {Directory for extracted files in \enquote{sandboxed} \TeX{} runs}
+\luavarset{testdir}   {builddir .. "/test"}   {Directory for running tests}
+\luavarset{typesetdir}{builddir .. "/doc"}    {Directory for building documentation}
+\luavarset{unpackdir} {builddir .. "/unpack"} {Directory for unpacking sources}
 \luavarseparator
 \luavarset{ctandir}{distribdir .. "/ctan"}{Directory for organising files for CTAN}
 \luavarset{tdsdir} {distribdir .. "/tds"} {Directory for organised files into TDS structure}

l3build.lua

@@ -70,11 +70,12 @@ testfiledir = testfiledir or "testfiles"
 testsuppdir = testsuppdir or testfiledir .. "/support"
 
 -- Structure within a development area
-distribdir = distribdir or maindir .. "/build/distrib"
-localdir   = localdir   or maindir .. "/build/local"
-testdir    = testdir    or maindir .. "/build/test"
-typesetdir = typesetdir or maindir .. "/build/doc"
-unpackdir  = unpackdir  or maindir .. "/build/unpacked"
+builddir   = builddir   or maindir .. "/build"
+distribdir = distribdir or builddir .. "/distrib"
+localdir   = localdir   or builddir .. "/local"
+testdir    = testdir    or builddir .. "/test"
+typesetdir = typesetdir or builddir .. "/doc"
+unpackdir  = unpackdir  or builddir .. "/unpacked"
 
 -- Substructure for CTAN release material
 ctandir = ctandir or distribdir .. "/ctan"
@@ -691,7 +692,7 @@ function tree(path, glob)
       for _, file in ipairs(filelist(dir, pattern)) do
         local fullpath = path .. "/" .. file
         if file ~= "." and file ~= ".." and
-          fullpath ~= maindir .. "/build" and
+          fullpath ~= builddir and
           (sub(pattern, 1, 1) == "."
             or sub(file, 1, 1) ~= ".")
         then