configure: Allow overriding --enable-debug flags setting
configure: Allow overriding --enable-debug flags setting

E.g. for using -ggdb3 or -g3 to enable debugger C macro expansion.
For example:

CFLAGS=-ggdb3 CXXFLAGS=-ggdb3 configure --enable-debug

linas/link-grammar,ampli/link-grammar,opencog/link-grammar,opencog/link-grammar,linas/link-grammar,linas/link-grammar,ampli/link-grammar,opencog/link-grammar,ampli/link-grammar,linas/link-grammar,linas/link-grammar,ampli/link-grammar,ampli/link-grammar,linas/link-grammar,opencog/link-grammar,linas/link-grammar,ampli/link-grammar,ampli/link-grammar,linas/link-grammar,opencog/link-grammar,linas/link-grammar,opencog/link-grammar,opencog/link-grammar,opencog/link-grammar,opencog/link-grammar,ampli/link-grammar,ampli/link-grammar