fix a few linux compilation warnings

pdfium.gyp

 {
   'variables': {
     'pdf_use_skia%': 0,
   },
   'target_defaults': {
     'defines' : [
       'FT2_BUILD_LIBRARY',
       '_FPDFSDK_LIB',
       '_NO_GDIPLUS_',  # workaround text rendering issues on Windows
       'OPJ_STATIC',
     ],
     'include_dirs': [
       'third_party/freetype/include',
     ],
     'conditions': [
       ['pdf_use_skia==1', {
         'defines': ['_SKIA_SUPPORT_'],
       }],
       ['OS=="linux"', {
         'conditions': [
           ['target_arch=="x64"', {
             'defines' : [ '_FX_CPU_=_FX_X64_', ],
-            'cflags': [ '-fPIC', ],
+            'cflags': [ '-fPIC', '-Wno-sign-compare'],
           }],
           ['target_arch=="ia32"', {
             'defines' : [ '_FX_CPU_=_FX_X86_', ],
           }],
         ],
       }],
     ],
     'msvs_disabled_warnings': [
       4005, 4018, 4146, 4333, 4345, 4267
     ],
@@ skipping 787 matching lines @@
         '<(DEPTH)'
       ],
       'sources': [
         'testing/fx_string_testhelpers.h',
         'testing/fx_string_testhelpers.cpp',
         'core/src/fxcrt/fx_basic_bstring_unittest.cpp',
       ],
     },
   ],
 }