Fix configure script on bash 2.0
../configure: line 11885: CFLAGS+= -Werror -Wextra: command not found
../configure: line 11886: CFLAGS+= -Wno-unused-parameter: command not found
../configure: line 11887: CFLAGS+= -Wno-missing-field-initializers: command not found
../configure: line 11888: CFLAGS+= -Wdeclaration-after-statement: command not found
../configure: line 11889: CFLAGS+= -Wmissing-declarations: command not found
../configure: line 11890: CFLAGS+= -Wredundant-decls: command not found
../configure: line 11891: CFLAGS+= -Wcast-align: command not found