Fix build breakage when libreoffice is installed.

The problem is that libreoffice installs its own copy of unopkg in
${PREFIX}/bin and that the openoffice build attempts to use this copy
of unopkg instead of the openoffice version which is in subdirectory
of ${WRKSRC}.

The reason is that the openoffice build expects to find its own copy
of unopkg by depending on having "." in its $PATH, but its $PATH has
${PREFIX}/bin before ".". Openoffice attempts to do the right thing
by first constructing $PATH by prepending "." and a small number of
other directories where it stashes executables used during the build
to the value of $PATH that it inherits from the environment. Things
go wrong when it tries to add the paths for ${CC}, perl, and java
to $PATH. If $PATH has /usr/bin before ${PREFIX}/bin, the openoffice
build finds the perl symlink in /usr/bin before it finds perl in
${PREFIX}/bin, so it prepends ${PREFIX}/bin to $PATH to try to ensure
that the correct version of perl will be found first. This moves
${PREFIX}/bin earlier in $PATH than ".".

The operation to put the path to ${CC} in $PATH has a different
problem. It uses the variable COMPATH for this, which is the dirname
of the patch to $CC, with the trailing /bin stripped off. That
results in /usr/local being added to $PATH, which is nonsensical,
though mostly harmless.

There are three fixes here:

• Always keep the parts of $PATH for the directories under ${WRKSRC}, including "." at the beginning of the path.

• Ignore symlinks to executables when deciding to prepend a directory to the path.

• Append "/bin" when using COMPATH so that the result points to the directory where ${CC} actually resides. There is actually another variable CC_PATH, but it has an extra trailing "/", so it doesn't match ${PREFIX}/bin.

Differential Revision: https://reviews.freebsd.org/D1958
Reviewed by: pfg
Approved by: mat (mentor)