Add finder for MPFR.

finders/FindMPFR.cmake

+find_path(MPFR_INCLUDE_DIR
+    NAMES mpfr.h
+    PATHS /usr/local/include
+    /usr/include
+    )
+
+find_library(MPFR_LIBRARY
+    NAMES mpfr
+    PATHS /usr/local/lib
+    /usr/lib
+    )
+
+include(FindPackageHandleStandardArgs)
+find_package_handle_standard_args(MPFR
+    REQUIRED_VARS MPFR_INCLUDE_DIR MPFR_LIBRARY)
+
+if(MPFR_FOUND AND NOT TARGET MPFR::MPFR)
+    add_library(MPFR::MPFR INTERFACE IMPORTED)
+    target_include_directories(MPFR::MPFR INTERFACE ${MPFR_INCLUDE_DIR})
+    target_link_libraries(MPFR::MPFR INTERFACE ${MPFR_LIBRARY})
+endif()
+
+mark_as_advanced(MPFR_INCLUDE_DIR)
+mark_as_advanced(MPFR_LIBRARY)
+