javacomp-script, javacomp: Remove support for javac versions < 1.6.
* m4/javacomp.m4 (gt_JAVACOMP): State that the minimum source_version
and the minimum target_version are 1.6. Map smaller values to 1.6.
Complain if the java version is < 1.6. Use 1.6 as default, instead of
1.1. Don't bother trying the -target option alone.
* lib/javacomp.h (compile_java_class): State that the minimum
source_version and the minimum target_version are 1.6.
* lib/javacomp.c (default_target_version): Complain if the java version
is < 1.6. Use 1.6 as default, instead of 1.1.
(SOURCE_VERSION_BOUND, source_version_index, get_goodcode_snippet,
get_failcode_snippet): Adjust to the new minimum source_version = 1.6.
(TARGET_VERSION_BOUND, target_version_index,
corresponding_classfile_version): Adjust to the new minimum
target_version = 1.6.
(get_source_version_for_javac): Remove function.
(is_envjavac_usable): Remove source_version_for_javac parameter. Don't
bother trying the -target option alone.
(is_javac_usable): Likewise.
(compile_java_class): Map source_version < 1.6 to 1.6. Map
target_version < 1.6 to 1.6. Use source_version instead of calling
get_source_version_for_javac.