Top makefile: add MAKE var and C option

Top makefile: add MAKE var and C option

[Alfonso S. Siciliano]

Hello,

Recently I proposed an update for src/makefile and the top makefile; now 
both committed. I omitted a description for the top makefile change. My 
mistake, I apologize.

The change is

all :
- cd src ; make
+ $(MAKE) -C src

clean :
- cd src ; make clean
+ $(MAKE) -C src clean

Description and rationale.

Change 1: Replace make with the $MAKE variable.
Why? GNU make replaces this variable with the utility name used to 
"perform" the makefile. This fixes naming problem because some OS names 
GNU make "gmake". Moreover I found a problem testing edbrowse with 
automatic building system (used by distros and OSs package builders). 
Usually they run make with the -j<number jobs> option to build in 
parallel. The $MAKE variable passes also -j to the makefile in 
src/makefile avoiding system warnings and faults. The $MAKE variable 
variable is documented in
<https://www.gnu.org/software/make/manual/html_node/MAKE-Variable.html>.

Change 2: Replace cd src with -C src.
It is just a sugar syntax, the -C option is documented in 
<https://www.gnu.org/software/make/manual/html_node/Recursion.html>.

Best Regards,
Alfonso