Prefer gawk over awk if both are available.

[LawnGnome]

This fixes rbenv help on Solaris 11.1.

[mislav]

I would like to hear more explanation. How is awk on Solaris broken?

[LawnGnome]

Output:

vagrant@solaris-vagrant:~/.rbenv$ rbenv help
Usage: rbenv <command> [<args>]

Some useful rbenv commands are:
awk: syntax error near line 13
awk: bailing out near line 13
awk: newline in string near line 24
awk: syntax error near line 13
awk: bailing out near line 13
awk: newline in string near line 24
awk: syntax error near line 13
awk: bailing out near line 13
awk: newline in string near line 24
awk: syntax error near line 13
awk: bailing out near line 13
awk: newline in string near line 24
awk: syntax error near line 13
awk: bailing out near line 13
awk: newline in string near line 24
awk: syntax error near line 13
awk: bailing out near line 13
awk: newline in string near line 24
awk: syntax error near line 13
awk: bailing out near line 13
awk: newline in string near line 24
awk: syntax error near line 13
awk: bailing out near line 13
awk: newline in string near line 24
awk: syntax error near line 13
awk: bailing out near line 13
awk: newline in string near line 24
awk: syntax error near line 13
awk: bailing out near line 13
awk: newline in string near line 24
awk: syntax error near line 13
awk: bailing out near line 13
awk: newline in string near line 24

See `rbenv help <command>' for information on a specific command.
For full documentation, see: https://github.com/sstephenson/rbenv#readme

The first error appears to be that Solaris awk doesn't support the /^( *$| )/ && reading_usage pattern syntax (it presumably wants a real regex rather than a global variable to check). I'm somewhat hazier on the second error — it's been some years since I did anything much with awk!

[mislav]

Gah! Why doesn't Solaris just have GNU utils as first-class citizens? Anyway, merged. Thanks!