[libc][MPFRWrapper] Provide a way to include MPFR header in downstream repos.

libc/utils/MPFRWrapper/MPFRUtils.cpp

 
 #include "utils/FPUtil/FPBits.h"
 #include "utils/FPUtil/TestHelpers.h"
 
 #include "llvm/ADT/StringExtras.h"
 #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 {
 namespace testing {
 namespace mpfr {
 
 class MPFRNumber {
   // A precision value which allows sufficiently large additional