Tag compilers that support/don't support amdlibm.

theano/gof/cmodule.py

@@ -1770,6 +1770,8 @@ class GCC_compiler(Compiler):
     # The equivalent flags of --march=native used by g++.
     march_flags = None
 
+    supports_amdlibm = True
+
     @staticmethod
     def version_str():
         return theano.config.cxx + " " + gcc_version_str

theano/sandbox/cuda/nvcc_compiler.py

@@ -131,6 +131,8 @@ def add_standard_rpath(rpath):
 
 
 class NVCC_compiler(Compiler):
+    supports_amdlibm = False
+
     @staticmethod
     def try_compile_tmp(src_code, tmp_prefix='', flags=(),
                         try_run=False, output=False):