Fix buildtime error caused by using alternate toolchain

[cross-cisco]

Addresses issue #2800, where a make $(shell command will fail if an alternate cpp is needed.
Without this change, using a cross-compile toolchain:

> make clean-nbase
In file included from <command-line>:31:                                       
./nmap.h:91:10: fatal error: assert.h: No such file or directory               
   91 | #include <assert.h>
      |          ^~~~~~~~~~
compilation terminated.
cd nbase && make clean
make[1]: Entering directory '.../nmap-7.94/nbase'
rm -f snprintf.o nbase_time.o nbase_str.o nbase_misc.o nbase_memalloc.o nbase_rnd.o nbase_addrset.o libnbase.a
make[1]: Leaving directory '.../nmap-7.94/nbase'
> echo $?
0
>

With this change:

> make clean-nbase
cd nbase && make clean
make[1]: Entering directory '.../nmap-7.94/nbase'
rm -f snprintf.o nbase_time.o nbase_str.o nbase_misc.o nbase_memalloc.o nbase_rnd.o nbase_addrset.o libnbase.a 
make[1]: Leaving directory '.../nmap-7.94/nbase'
>