Fix a new bug in GNU Make (#2447)

CXX should not be set to mpiicpc if it's not an MPI build.  This is a new
bug introduced by me in #2435.

Author: WeiqunZhang

Tools/GNUMake/sites/Make.unknown

@@ -4,6 +4,7 @@
 
 os_type := $(shell uname)
 
+ifeq ($(USE_MPI),TRUE)
 ifeq ($(USE_DPCPP),TRUE)
    ifeq ($(shell mpiicpc -show > /dev/null 2>&1; echo $$?), 0)
       CC  := mpiicc
@@ -14,6 +15,7 @@ ifeq ($(USE_DPCPP),TRUE)
       NO_MPI_CHECKING = TRUE
    endif
 endif
+endif
 
 ifneq ($(NO_CONFIG_CHECKING),TRUE)