GH-48340: [R] respected MAKEFLAGS (#48341)

### Rationale for this change
Respect the `MAKEFLAGS` that someone has set.

Resolves #48340

### What changes are included in this PR?
Respect

### Are these changes tested?
No, though we should see speedups in various places.

### Are there any user-facing changes?
More respect.
* GitHub Issue: #48340

Author: jonkeane

r/tools/nixlibs.R

@@ -540,6 +540,12 @@ build_libarrow <- function(src_dir, dst_dir) {
   if (makeflags == "") {
     makeflags <- sprintf("-j%s", ncores)
     Sys.setenv(MAKEFLAGS = makeflags)
+  } else {
+    # Extract -j value from existing MAKEFLAGS if present
+    j_match <- regmatches(makeflags, regexpr("-j\\s*([0-9]+)", makeflags, perl = TRUE))
+    if (length(j_match) > 0) {
+      ncores <- as.integer(sub("-j\\s*", "", j_match, perl = TRUE))
+    }
   }
   if (!quietly) {
     lg("Building with MAKEFLAGS=%s", makeflags)