Prevent incpath to spill into libpth

Prevent incpath to spill into libpth

• Use realpath if available
• This might catch more duplicate paths
• Only include real existing paths
• Filter inc paths out of libpth

Before:
incpth: /usr/lib64/gcc/x86_64-suse-linux/7/include /usr/local/include /usr/lib64/gcc/x86_64-suse-linux/7/include-fixed /usr/lib64/gcc/x86_64-suse-linux/7/../../../../x86_64-suse-linux/include /usr/include
libpth: /usr/local/lib /usr/lib64/gcc/x86_64-suse-linux/7/include-fixed /usr/lib64/gcc/x86_64-suse-linux/7/../../../../x86_64-suse-linux/lib /usr/lib /pro/local/lib /lib/../lib64 /usr/lib/../lib64 /lib /lib64 /usr/lib64 /usr/local/lib64

After:
incpth: /usr/lib64/gcc/x86_64-suse-linux/7/include /usr/local/include /usr/lib64/gcc/x86_64-suse-linux/7/include-fixed /usr/x86_64-suse-linux/include /usr/include
libpth: /usr/local/lib /usr/x86_64-suse-linux/lib /usr/lib /pro/local/lib /lib64 /usr/lib64 /lib /usr/local/lib64

(Backported from perl.git d3144c9253d3244cd259a0ae4c0fe81519bf5b53 to
 metaconfig.git by Niko Tyni)