diff --git a/src/build/llvmPackages.ts b/src/build/llvmPackages.ts
index 76de635..390ed3e 100644
--- a/src/build/llvmPackages.ts
+++ b/src/build/llvmPackages.ts
@@ -57,11 +57,7 @@ export interface LLVMPackageOptions {
     /**
      * Build directories suitable for these three packages.
      */
-    buildDirs: {
-        llvm: string;
-        cflang: string;
-        libpgmath: string;
-    };
+    buildDir: (packageName: 'llvm' | 'libpgmath' | 'cflang') => string,
 
     platform: PackagePlatform;
 };
@@ -70,7 +66,7 @@ export function llvmPackages({
     src,
     installPrefix,
     platform,
-    buildDirs,
+    buildDir,
     cmakeDefinitionOverrides,
     spawnOverrides,
 }: LLVMPackageOptions & LLVMPackageOverrides): PackageTask[] {
@@ -110,7 +106,7 @@ export function llvmPackages({
     });
 
     function mkLLVMPackage(): PackageTask {
-        const build = buildDirs.llvm;
+        const build = buildDir('llvm');
         return {
             name: "llvm",
             platform,
@@ -145,7 +141,7 @@ export function llvmPackages({
     }
 
     function mkLibpgmathPackage(): PackageTask {
-        const build = buildDirs.libpgmath;
+        const build = buildDir('libpgmath');
         return {
             name: "libpgmath",
             configurePhase: async () => {
@@ -174,7 +170,7 @@ export function llvmPackages({
     }
 
     function mkCflangPackage(): PackageTask {
-        const build = buildDirs.cflang;
+        const build = buildDir('cflang');
         return {
             name: "cflang",
             configurePhase: async () => {