diff --git a/externals/CMakeLists.txt b/externals/CMakeLists.txt
index 6f2dfff5db9c2592bc1ec062bcbe7b93cafe5879..773c8e8bf2595f5d0032eacbfb64f88a884b4025 100644
--- a/externals/CMakeLists.txt
+++ b/externals/CMakeLists.txt
@@ -143,7 +143,7 @@ LCGPackage_Add(
 LCGPackage_Add(
   mpfr
   URL ${GenURL}/mpfr-${mpfr_native_version}.tar.gz
-  CONFIGURE_COMMAND ./configure --prefix=<INSTALL_DIR> --with-gmp=${gmp_home}
+  CONFIGURE_COMMAND ./configure --prefix=<INSTALL_DIR> IF NOT ${gmp_home} STREQUAL TakenFromSystem THEN --with-gmp=${gmp_home} ENDIF
   BUILD_COMMAND ${MAKE}
   INSTALL_COMMAND ${MAKE} install
   BUILD_IN_SOURCE 1
@@ -153,7 +153,7 @@ LCGPackage_Add(
 LCGPackage_Add(
   mpfi
   URL ${GenURL}/mpfi-${mpfi_native_version}.tar.gz
-  CONFIGURE_COMMAND ./configure --prefix=<INSTALL_DIR> --with-gmp=${gmp_home} --with-mpfr=${mpfr_home}
+  CONFIGURE_COMMAND ./configure --prefix=<INSTALL_DIR> IF NOT ${gmp_home} STREQUAL TakenFromSystem THEN --with-gmp=${gmp_home} ENDIF --with-mpfr=${mpfr_home}
   BUILD_COMMAND ${MAKE}
   INSTALL_COMMAND ${MAKE} install
   BUILD_IN_SOURCE 1
@@ -1786,7 +1786,7 @@ LCGPackage_Add(
 LCGPackage_Add(
   fplll
   URL ${GenURL}/fplll-${fplll_native_version}.tar.gz
-  CONFIGURE_COMMAND ./configure --prefix=<INSTALL_DIR> --with-mpfr=${mpfr_home} --with-gmp=${gmp_home}
+  CONFIGURE_COMMAND ./configure --prefix=<INSTALL_DIR> --with-mpfr=${mpfr_home} IF NOT ${gmp_home} STREQUAL TakenFromSystem THEN --with-gmp=${gmp_home} ENDIF
   BUILD_COMMAND ${MAKE}
   INSTALL_COMMAND ${MAKE} install
   BUILD_IN_SOURCE 1
@@ -1798,7 +1798,7 @@ LCGPackage_Add(
   sollya
   URL ${GenURL}/sollya-${sollya_native_version}.tar.bz2
   ENVIRONMENT ${library_path}=${mpfi_home}/lib:\$ENV{${library_path}}
-  CONFIGURE_COMMAND ./configure --prefix=<INSTALL_DIR> --with-gmp=${gmp_home} --with-mpfr=${mpfr_home} --with-mpfi=${mpfi_home} --with-xml2=${libxml2_home} --with-fplll=${fplll_home} CC=${CMAKE_C_COMPILER} CXX=${CMAKE_CXX_COMPILER}
+  CONFIGURE_COMMAND ./configure --prefix=<INSTALL_DIR>  IF NOT ${gmp_home} STREQUAL TakenFromSystem THEN --with-gmp=${gmp_home} ENDIF --with-mpfr=${mpfr_home} --with-mpfi=${mpfi_home} --with-xml2=${libxml2_home} --with-fplll=${fplll_home} CC=${CMAKE_C_COMPILER} CXX=${CMAKE_CXX_COMPILER}
   BUILD_COMMAND ${MAKE}
   INSTALL_COMMAND ${MAKE} install
   BUILD_IN_SOURCE 1
diff --git a/pyexternals/CMakeLists.txt b/pyexternals/CMakeLists.txt
index ff4fe52b86c6d2a68d98c37b4662cde128038262..72a47da50b47b388b74f6d3ecf505d4d1e2b8f2b 100644
--- a/pyexternals/CMakeLists.txt
+++ b/pyexternals/CMakeLists.txt
@@ -404,7 +404,7 @@ LCGPackage_Add(
   URL ${GenURL}/pythonsollya-${pythonsollya_native_version}.tgz
   CONFIGURE_COMMAND <VOID>
   BUILD_COMMAND ${MAKE} PREFIX=<INSTALL_DIR> SOLLYA_DIR=${sollya_home}
-                MPFI_DIR=${mpfi_home} MPFR_DIR=${mpfr_home} GMP_DIR=${gmp_home}
+                MPFI_DIR=${mpfi_home} MPFR_DIR=${mpfr_home} IF NOT ${gmp_home} STREQUAL TakenFromSystem THEN GMP_DIR=${gmp_home} ENDIF
                 PYTHON=${Python_home}/bin/python PIP=${pip_home}/bin/pip
 #  INSTALL_COMMAND ${MAKE} PREFIX=<INSTALL_DIR> -j1 install
   INSTALL_COMMAND ${PYTHON} setup.py install ${PySetupOptions}