diff --git a/libc/utils/MPFRWrapper/MPFRUtils.cpp b/libc/utils/MPFRWrapper/MPFRUtils.cpp
--- a/libc/utils/MPFRWrapper/MPFRUtils.cpp
+++ b/libc/utils/MPFRWrapper/MPFRUtils.cpp
@@ -15,10 +15,20 @@
 #include "llvm/ADT/StringRef.h"
 
 #include <memory>
-#include <mpfr.h>
 #include <stdint.h>
 #include <string>
 
+#ifdef CUSTOM_MPFR_INCLUDER
+// Some downstream repos are monoliths carrying MPFR sources in their third
+// party directory. In such repos, including the MPFR header as
+// `#include <mpfr.h>` is either disallowed or not possible. If that is the
+// case, a file named `CustomMPFRIncluder.h` should be added through which the
+// MPFR header can be included in manner allowed in that repo.
+#include "CustomMPFRIncluder.h"
+#else
+#include <mpfr.h>
+#endif
+
 template <typename T> using FPBits = __llvm_libc::fputil::FPBits<T>;
 
 namespace __llvm_libc {