Fix "--enable-code-coverage" debug build

When --enable-code-coverage is provided it should not result
in NDEBUG being defined.  This is controlled by --enable-debug.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #6674

config/ax_code_coverage.m4

@@ -124,7 +124,7 @@ AC_DEFUN([AX_CODE_COVERAGE],[
 
 		dnl Build the code coverage flags
 		dnl Define CODE_COVERAGE_LDFLAGS for backwards compatibility
-		CODE_COVERAGE_CPPFLAGS="-DNDEBUG"
+		CODE_COVERAGE_CPPFLAGS=""
 		CODE_COVERAGE_CFLAGS="-O0 -g -fprofile-arcs -ftest-coverage"
 		CODE_COVERAGE_CXXFLAGS="-O0 -g -fprofile-arcs -ftest-coverage"
 		CODE_COVERAGE_LIBS="-lgcov"