diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index c433953020f01137da38d37370d809693529cbd4..aaf567e14c67a326a7a7351f081f3818a023a896 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -26,7 +26,7 @@ Examples:
 Python 3 without small-integer opt:
   script: |
     curl -L -O -k https://gitlab.tiker.net/inducer/ci-support/raw/main/build-and-test-py-project.sh
-    ./configure.py --no-use-imath-sio
+    export PROJECT_INSTALL_FLAGS="--config-settings=cmake.define.USE_IMATH_SIO=OFF"
     . ./build-and-test-py-project.sh
   tags:
   - python3
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 941ae86cd5f46b3a8aa3ab16237a518bd640fc35..bafa6e447fbbf1eec3733ef5f652c83f8d235db2 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -18,12 +18,12 @@ execute_process(
 list(APPEND CMAKE_PREFIX_PATH "${NB_DIR}")
 find_package(nanobind CONFIG REQUIRED)
 
-option(USE_SHIPPED_ISL "Use shipped ISL" 1)
-option(USE_SHIPPED_IMATH "Use shipped IMATH" 1)
-option(USE_IMATH_FOR_MP "Use IMATH for multiprecision arithmetic" 1)
-option(USE_IMATH_SIO "Use IMATH small-integer optimization" 1)
-option(USE_GMP_FOR_MP "Use GMP" 0)
-option(USE_BARVINOK "Use Barvinok (beware of GPL license)" 0)
+option(USE_SHIPPED_ISL "Use shipped ISL" ON)
+option(USE_SHIPPED_IMATH "Use shipped IMATH" ON)
+option(USE_IMATH_FOR_MP "Use IMATH for multiprecision arithmetic" ON)
+option(USE_IMATH_SIO "Use IMATH small-integer optimization" ON)
+option(USE_GMP_FOR_MP "Use GMP" OFF)
+option(USE_BARVINOK "Use Barvinok (beware of GPL license)" OFF)
 
 if(USE_SHIPPED_ISL)
     if(USE_BARVINOK)
@@ -123,7 +123,11 @@ if(USE_SHIPPED_ISL)
         ${CMAKE_SOURCE_DIR}/isl/include
         ${CMAKE_SOURCE_DIR}/isl
     )
-    if(USE_IMATH_FOR_MP)
+    if(USE_GMP_FOR_MP)
+        list(APPEND ISL_SOURCES
+            isl/isl_val_gmp.c
+        )
+    elseif(USE_IMATH_FOR_MP)
         if(USE_SHIPPED_IMATH)
             list(APPEND ISL_SOURCES
                 isl/isl_imath.c
@@ -131,13 +135,17 @@ if(USE_SHIPPED_ISL)
                 isl/imath/imrat.c
                 isl/imath/gmp_compat.c
             )
-            list(APPEND ISL_INC_DIRS isl/imath)
+            list(APPEND ISL_INC_DIRS ${CMAKE_SOURCE_DIR}/isl/imath)
         endif()
         if(USE_IMATH_SIO)
             list(APPEND ISL_SOURCES
                 isl/isl_int_sioimath.c
                 isl/isl_val_sioimath.c
             )
+        else()
+            list(APPEND ISL_SOURCES
+                isl/isl_val_imath.c
+            )
         endif()
     endif()
 else()