Fix Compilation Issues in Win-builer

[brodieG]

Log:

* installing *source* package 'diffobj' ...
** libs

*** arch - i386
d:/Compiler/gcc-4.9.3/mingw_32/bin/gcc  -I"D:/RCompile/recent/R-3.3.1/include" -DNDEBUG     -I"d:/Compiler/gcc-4.9.3/local330/include"     -O3 -Wall  -std=gnu99 -mtune=core2 -c diff.c -o diff.o
diff.c: In function '_find_faux_snake':
diff.c:204:17: warning: variable 'k_r' set but not used [-Wunused-but-set-variable]
   int x_r, y_r, k_r;
                 ^
diff.c: At top level:
diff.c:613:1: warning: 'diff' redeclared without dllimport attribute: previous dllimport ignored [-Wattributes]
 diff(SEXP a, int aoff, int n, SEXP b, int boff, int m,
 ^
diff.c: In function 'diff':
diff.c:660:7: warning: assuming signed overflow does not occur when assuming that (X - c) > X is always false [-Wstrict-overflow]
     if(boff + y < boff + y - 1 || aoff + x < aoff + x - 1)
       ^
diff.c:660:32: warning: assuming signed overflow does not occur when assuming that (X - c) > X is always false [-Wstrict-overflow]
     if(boff + y < boff + y - 1 || aoff + x < aoff + x - 1)
                                ^
d:/Compiler/gcc-4.9.3/mingw_32/bin/gcc  -I"D:/RCompile/recent/R-3.3.1/include" -DNDEBUG     -I"d:/Compiler/gcc-4.9.3/local330/include"     -O3 -Wall  -std=gnu99 -mtune=core2 -c diffobj.c -o diffobj.o
d:/Compiler/gcc-4.9.3/mingw_32/bin/gcc  -I"D:/RCompile/recent/R-3.3.1/include" -DNDEBUG     -I"d:/Compiler/gcc-4.9.3/local330/include"     -O3 -Wall  -std=gnu99 -mtune=core2 -c init.c -o init.o
d:/Compiler/gcc-4.9.3/mingw_32/bin/gcc -shared -s -static-libgcc -o diffobj.dll tmp.def diff.o diffobj.o init.o -Ld:/Compiler/gcc-4.9.3/local330/lib/i386 -Ld:/Compiler/gcc-4.9.3/local330/lib -LD:/RCompile/recent/R-3.3.1/bin/i386 -lR
diffobj.o:diffobj.c:(.text+0xc6): undefined reference to `_imp__diff'
collect2.exe: error: ld returned 1 exit status
no DLL was created
ERROR: compilation failed for package 'diffobj'
* removing 'd:/RCompile/CRANguest/R-release/lib/diffobj'