pkg-config support was added very recently to gmp, so a lot of platforms
we want to support won't find gmp this way.
if (PKG_CONFIG_FOUND)
pkg_check_modules(GMP gmp)
-else()
+endif()
+
+# Only very recent versions of gmp has pkg-config support, so we have to
+# fall back on a more classical search
+if(NOT GMP_FOUND)
find_path(GMP_INCLUDE_DIRS NAMES gmp.h PATH_SUFFIXES)
find_library(GMP_LIBRARIES NAMES gmp)
find_package_handle_standard_args(GMP DEFAULT_MSG GMP_LIBRARIES GMP_INCLUDE_DIRS)