Refactor local `execvpe(3)` implementation

The previous code was prone to conflicts with when the platform happens
to expose a `execvpe(3)` implementation in its libc.

This commit renames the internal implementation to `__hsunix_execvpe` as
well as adding an autoconf-detection for the presence of `execvpe(3)`,
in which case `__hsunix_execvpe()` forwards the call to `execvpe(3)`.

Moreover, the code has been cleaned up to remove likely bitrotted CPP
conditionals.

This should fix #22

(This also partially addresses #11 on platforms which have a
libc-provided `execvpe(3)`)

1 files changed, 1 insertions, 0 deletions

diff --git a/unix.cabal b/unix.cabal
index ddd95de..69470ba 100644
--- a/unix.cabal
+++ b/unix.cabal
@@ -127,3 +127,4 @@ library
         cbits/HsUnix.c
         cbits/dirUtils.c
         cbits/execvpe.c
+        cbits/ghcrts.c