Fixed some build issues

Hi,

I fixed some problems with the build:

- out-of source build

- An compilation error in esch.c with clang in c++ mode:

/bin/sh ../libtool  --tag=CC   --mode=compile clang++ -DHAVE_CONFIG_H -I. -I../../esch -I..  -I../../util -I../../api   -g -O2 -Wall -W -Wcast-qual -Wpointer-arith -Wcast-align -Wno-long-long -pedantic -Wshadow -Wwrite-strings -Wredundant-decls -Wundef -Wconversion -MT esch.lo -MD -MP -MF .deps/esch.Tpo -c -o esch.lo ../../esch/esch.c
libtool: compile:  clang++ -DHAVE_CONFIG_H -I. -I../../esch -I.. -I../../util -I../../api -g -O2 -Wall -W -Wcast-qual -Wpointer-arith -Wcast-align -Wno-long-long -pedantic -Wshadow -Wwrite-strings -Wredundant-decls -Wundef -Wconversion -MT esch.lo -MD -MP -MF .deps/esch.Tpo -c ../../esch/esch.c  -fPIC -DPIC -o .libs/esch.o
clang: warning: treating 'c' input as 'c++' when in C++ mode, this behavior is deprecated
../../esch/esch.c:138:6: error: use of undeclared identifier 'memcpy'
     memcpy(esparents[0].parameters, x, nparameters * sizeof(double));
     ^
../../esch/esch.c:167:9: error: use of undeclared identifier 'memcpy'
               memcpy(x, esparents[id].parameters, 
               ^
../../esch/esch.c:220:7: error: use of undeclared identifier 'memcpy'
                    memcpy(x, esoffsprings[id].parameters, 
                    ^
3 errors generated.
make[2]: *** [esch.lo] Error 1
make[2]: Leaving directory `/home/xantares/projects/nlopt/build/esch'
make[1]: *** [all-recursive] Error 1

- I also noticed that autogen.sh calls configure, that's not common practice, what if I want to configure elsewhere ? Could we clean it, and add an autogen+configure.sh instead ?

x.

Could we clean