Enable compilation with C90 compilers

Enable compilation with C90 compilers

* configure.ac: Try compiling Guile headers: they don't work with C90.
* maintMakefile: Simplify config checks via target-specific variables.
* src/makeint.h: Use ATTRIBUTE rather than defining __attribute__,
as that causes compile issues with system headers.
(ENUM_BITFIELD): Don't use enum bitfields in ANSI mode.
* src/main.c: Use ATTRIBUTE instead of __attribute__.
* src/job.h: Ditto.
* src/file.c: Don't define variables inside for loops.
* src/rule.c: Ditto.
* src/dep.h (SI): Only use static inline in non-ANSI mode.