guix: move static-libc++ into CMAKE_EXE_LINKER_FLAGS flags

Make it clearer that we are only applying this to executables.

contrib/guix/libexec/build.sh

@@ -225,10 +225,15 @@ esac
 
 # LDFLAGS
 case "$HOST" in
-    *linux*)  HOST_LDFLAGS="-Wl,--as-needed -Wl,--dynamic-linker=$glibc_dynamic_linker -static-libstdc++ -Wl,-O2" ;;
+    *linux*)  HOST_LDFLAGS="-Wl,--as-needed -Wl,--dynamic-linker=$glibc_dynamic_linker -Wl,-O2" ;;
     *mingw*)  HOST_LDFLAGS="-Wl,--no-insert-timestamp" ;;
 esac
 
+# EXE FLAGS
+case "$HOST" in
+    *linux*)  CMAKE_EXE_LINKER_FLAGS="-DCMAKE_EXE_LINKER_FLAGS=${HOST_LDFLAGS} -static-libstdc++" ;;
+esac
+
 mkdir -p "$DISTSRC"
 (
     cd "$DISTSRC"
@@ -243,7 +248,8 @@ mkdir -p "$DISTSRC"
           --toolchain "${BASEPREFIX}/${HOST}/toolchain.cmake" \
           -DWITH_CCACHE=OFF \
           -Werror=dev \
-          ${CONFIGFLAGS}
+          ${CONFIGFLAGS} \
+          "${CMAKE_EXE_LINKER_FLAGS}"
 
     # Build Bitcoin Core
     cmake --build build -j "$JOBS" ${V:+--verbose}