Use -print-prog-name instead of -print-file-name in gcc spkg-configure.m4

[isuruf]

Here's what I get

~/sage$ echo $CC
/projects/66d93023-00f0-4c12-8a25-5d6d4e486740/sage-build/bin/x86_64-conda_cos6-linux-gnu-cc

~/sage$ $CC -print-prog-name=ld
/projects/66d93023-00f0-4c12-8a25-5d6d4e486740/sage-build/bin/../lib/gcc/x86_64-conda_cos6-linux-gnu/7.3.0/../../../../x86_64-conda_cos6-linux-gnu/bin/ld

~/sage$ $CC -print-file-name=ld
ld

~/sage$ echo $LD
/projects/66d93023-00f0-4c12-8a25-5d6d4e486740/sage-build/bin/x86_64-conda_cos6-linux-gnu-ld

Therefore -print-file-name is clearly not what we need.
-print-prog-name gives a path with ..s in it.

We need to check that the two files are equivalent using -ef which checks that the files are the same.

CC: @embray @dimpase

Component: build: configure

Author: Isuru Fernando

Branch/Commit: dec073b

Reviewer: Dima Pasechnik

Issue created by migration from https://trac.sagemath.org/ticket/28317

[dimpase]

comment:3

looks good.

[dimpase]

Reviewer: Dima Pasechnik

[vbraun]

Changed branch from u/isuruf/gcc-prog to dec073b