gdb/compile/compile.c: Check return value of 'system' to avoid compiler warning

gdb/compile/compile.c: Check return value of 'system' to avoid compiler warning

Under Ubuntu 12, we need to check the return value of system(), or the
compiler warns:

  gcc -g -O2   -I. -I../../binutils-gdb/gdb -I../../binutils-gdb/gdb/common -I../../binutils-gdb/gdb/config -DLOCALEDIR="\"/usr/local/share/locale\"" -DHAVE_CONFIG_H -I../../binutils-gdb/gdb/../include/opcode -I../../binutils-gdb/gdb/../opcodes/.. -I../../binutils-gdb/gdb/../readline/.. -I../bfd -I../../binutils-gdb/gdb/../bfd -I../../binutils-gdb/gdb/../include -I../libdecnumber -I../../binutils-gdb/gdb/../libdecnumber  -I../../binutils-gdb/gdb/gnulib/import -Ibuild-gnulib/import   -DTUI=1  -Wall -Wdeclaration-after-statement -Wpointer-arith -Wpointer-sign -Wno-unused -Wunused-value -Wunused-function -Wno-switch -Wno-char-subscripts -Wmissing-prototypes -Wdeclaration-after-statement -Wempty-body -Wmissing-parameter-type -Wold-style-declaration -Wold-style-definition -Wformat-nonliteral -Werror -c -o compile.o -MT compile.o -MMD -MP -MF .deps/compile.Tpo ../../binutils-gdb/gdb/compile/compile.c
  ../../binutils-gdb/gdb/compile/compile.c: In function ‘do_rmdir’:
  ../../binutils-gdb/gdb/compile/compile.c:175:10: error: ignoring return value of ‘system’, declared with attribute warn_unused_result [-Werror=unused-result]
  cc1: all warnings being treated as errors
  make[2]: *** [compile.o] Error 1
  make[2]: Leaving directory `/upstream/build-binutils-s390/gdb'
  make[1]: *** [all-gdb] Error 2
  make[1]: Leaving directory `/upstream/build-binutils-s390'
  make: *** [all] Error 2

Also, 'zap' is leaking.

2015-01-09  Chen Gang  <gang.chen.5i5j@gmail.com>
	    Pedro Alves  <palves@redhat.com>

	* compile/compile.c: Include "gdb_wait.h".
	(do_rmdir): Check return value, and free 'zap'.