Avoid overriding user defined Makevars files

If a user has setup a Makevars file we should use the settings in that
directly rather than overriding them

NEWS.md

@@ -1,5 +1,8 @@
 # Development
 
+* `compile_dll()` now does not supply compiler flags if there is an existing
+  user defined Makevars file.
+
 * `local_build_tools()` function added to provide a deferred equivalent to
   `with_build_tools()`. So you can add rtools to the PATH until the end of a
   function body.

R/compile-dll.R

@@ -31,15 +31,27 @@ compile_dll <- function(path = ".", quiet = FALSE) {
   install_dir <- tempfile("devtools_install_")
   dir.create(install_dir)
 
-  withr::with_makevars(compiler_flags(TRUE), assignment = "+=", {
+  # If the user has a makevars file just use that
+  if (length(makevars_user()) > 0) {
     install_min(
       path,
       dest = install_dir,
       components = "libs",
       args = if (needs_clean(path)) "--preclean",
       quiet = quiet
     )
-  })
+  } else {
+    # Otherwise set makevars for fast development / debugging
+    withr::with_makevars(compiler_flags(TRUE), assignment = "+=", {
+      install_min(
+        path,
+        dest = install_dir,
+        components = "libs",
+        args = if (needs_clean(path)) "--preclean",
+        quiet = quiet
+        )
+    })
+  }
 
   invisible(dll_path(file.path(install_dir, pkg_name(path))))
 }

R/utils.R

@@ -22,3 +22,32 @@ is_windows <- function() {
 is_dir <- function(x) {
   isTRUE(file.info(x)$isdir)
 }
+
+# This is tools::makevars_user, provided here for backwards compatibility with older versions of R
+makevars_user <- function () {
+  m <- character()
+  if (.Platform$OS.type == "windows") {
+    if (!is.na(f <- Sys.getenv("R_MAKEVARS_USER", NA_character_))) {
+      if (file.exists(f))
+        m <- f
+    }
+    else if ((Sys.getenv("R_ARCH") == "/x64") && file.exists(f <- path.expand("~/.R/Makevars.win64")))
+      m <- f
+    else if (file.exists(f <- path.expand("~/.R/Makevars.win")))
+      m <- f
+    else if (file.exists(f <- path.expand("~/.R/Makevars")))
+      m <- f
+  }
+  else {
+    if (!is.na(f <- Sys.getenv("R_MAKEVARS_USER", NA_character_))) {
+      if (file.exists(f))
+        m <- f
+    }
+    else if (file.exists(f <- path.expand(paste0("~/.R/Makevars-",
+            Sys.getenv("R_PLATFORM")))))
+      m <- f
+    else if (file.exists(f <- path.expand("~/.R/Makevars")))
+      m <- f
+  }
+  m
+}