Error linking PostgreSQL custom C++ project in Visual Studio

I wrote a series of custom C PostgreSQL functions in Notepad++. I used the VS2015 x64 Native Tools command line utility to compile the code and linked it. No problems and worked perfectly.

But as much fun as coding in Notepad++ is, I decided to create a project in Visual Studio. After several hours of getting configurations correct, I can get the project to compile, but not link.

The specific errors are:

Error LNK2019 unresolved external symbol "double __cdecl DatumGetFloat8(unsigned __int64)" (?DatumGetFloat8@@YAN_K@Z) referenced in function "unsigned __int64 __cdecl wrapf64(struct FunctionCallInfoData *)" (?wrapf64@@YA_KPEAUFunctionCallInfoData@@@Z) ... Error LNK2019 unresolved external symbol "unsigned __int64 __cdecl Float8GetDatum(double)" (?Float8GetDatum@@YA_KN@Z) referenced in function "unsigned __int64 __cdecl wrapf64(struct FunctionCallInfoData *)" (?wrapf64@@YA_KPEAUFunctionCallInfoData@@@Z) ...

The symptom appears to be in fmgr.h (comments by me):

/* these macros hide the pass-by-reference-ness of the datatype: */
#define PG_GETARG_FLOAT4(n)  DatumGetFloat4(PG_GETARG_DATUM(n))  // missing a definition
#define PG_GETARG_FLOAT8(n)  DatumGetFloat8(PG_GETARG_DATUM(n))  // missing a definition
#define PG_GETARG_INT64(n)   DatumGetInt64(PG_GETARG_DATUM(n))

Looking in postgres.h we see:

#ifdef USE_FLOAT8_BYVAL
extern float8 DatumGetFloat8(Datum X);
#else
#define DatumGetFloat8(X) (* ((float8 *) DatumGetPointer(X)))
#endif

There is no #define. Adding one doesn't resolve the issue.

By contrast:

#ifdef USE_FLOAT8_BYVAL
#define DatumGetInt64(X) ((int64) GET_8_BYTES(X))
#else
#define DatumGetInt64(X) (* ((int64 *) DatumGetPointer(X)))
#endif

This is the unedited source code of PostgreSQL and it worked from the command line utilities. What's the problem inside VS? Or, should I say, why did it work in the command line linker when the #define is missing?

Any suggestions?

One other question I have is my original code had abs(double ...) but inside VS I have to use fabs(double ...). Is this a C vs C++ point? I couldn't find a clear answer in MSDN.