[bugfix] Fixes GMP test.

Previoulsy CMake always fails to find the __gmpz_abs symbol on my machine, even though it is clearly there.
The test used check_symbol_exists. According to the documentation, check_symbol_exists tries to include the
second argument into a test program to check whether there is a prototype for the first argument, so as the
second argument is a binary, that can't really work... This patch switches over to use check_library_exists.

cmake/modules/FindGMP.cmake

@@ -46,7 +46,7 @@ find_library(GMPXX_LIB gmpxx)
 # check if library works
 if(GMP_LIB AND GMPXX_LIB)
   include(CheckSymbolExists)
-  check_symbol_exists(__gmpz_abs ${GMP_LIB} GMPXX_LIB_WORKS)
+  check_library_exists(${GMP_LIB} __gmpz_abs "" GMPXX_LIB_WORKS)
 endif(GMP_LIB AND GMPXX_LIB)
 cmake_pop_check_state()
 
@@ -88,4 +88,4 @@ if(HAVE_GMP)
   foreach(dir ${GMP_INCLUDE_DIR})
     set_property(GLOBAL APPEND PROPERTY ALL_PKG_FLAGS "-I${dir}")
   endforeach()
-endif()
\ No newline at end of file
+endif()