circleci: Build with in-tree GMP on Darwin

Fixes #15404.

Author: bgamari

.circleci/config.yml

@@ -33,7 +33,7 @@ aliases:
   - &configure_unix
     run:
       name: Configure
-      command: ./configure
+      command: ./configure $CONFIGURE_OPTS
   - &configure_unix_32
     run:
       name: Configure
@@ -140,6 +140,8 @@ jobs:
       # Only Sierra and onwards supports clock_gettime. See #12858
       ac_cv_func_clock_gettime: "no"
       GHC_COLLECTOR_FLAVOR: x86_64-darwin
+      # Build with in-tree GMP since this isn't available on OS X by default.
+      CONFIGURE_OPTS: --with-intree-gmp
       <<: *buildenv
     steps:
       - checkout