Merge #315: cmake: Fix prefix maps for Guix builds

f32f42d fixup! cmake: Migrate Guix build scripts to CMake (Hennadii Stepanov)

Pull request description:

  Fixes #297.

  CMake uses absolute paths for source files when invoking a compiler. This change makes the resulting Guix binaries location independent.

  The resulting behaviour mirrors that of the master branch.

  ---

  NOTE. On the master branch, `HOST_CFLAGS+=" -ffile-prefix-map=${PWD}=."` evaluates to `HOST_CFLAGS+=" -ffile-prefix-map=/bitcoin=."`, which effectively does nothing because there is no such a path among source files being compiled. Binaries appear file path independent because file paths relative to the source directory are used when invoking a compiler.

ACKs for top commit:
  m3dwards:
    ACK f32f42d

Tree-SHA512: 790f010f22394c81f0e3d476934ee67390f8532a5b34f168b31621a56c978ef82563542cbbfc8b583cb38869169f291e814c7820ad86aef3c567b256b8a3c8fd

contrib/guix/libexec/build.sh

@@ -212,7 +212,7 @@ CONFIGFLAGS="-DREDUCE_EXPORTS=ON -DBUILD_BENCH=OFF -DBUILD_GUI_TESTS=OFF -DBUILD
 HOST_CFLAGS="-O2 -g"
 HOST_CFLAGS+=$(find /gnu/store -maxdepth 1 -mindepth 1 -type d -exec echo -n " -ffile-prefix-map={}=/usr" \;)
 case "$HOST" in
-    *linux*)  HOST_CFLAGS+=" -ffile-prefix-map=${PWD}=." ;;
+    *linux*)  HOST_CFLAGS+=" -ffile-prefix-map=${DISTSRC}/src=." ;;
     *mingw*)  HOST_CFLAGS+=" -fno-ident" ;;
     *darwin*) unset HOST_CFLAGS ;;
 esac