thirdparty/*.sh: add comments on POSIX-compliant scripts

thirdparty/configure-native-deps.sh

@@ -1,6 +1,11 @@
 #!/bin/sh
 # Use BSD/Linux system dependencies where possible, but take into account different .so names.
 
+####
+# This file must stay /bin/sh and POSIX compliant for BSD portability.
+# Copy-paste the entire script into http://shellcheck.net to check.
+####
+
 locations="/lib /lib64 /usr/lib /usr/lib64 /usr/lib/i386-linux-gnu /usr/lib/x86_64-linux-gnu /usr/local/lib /opt/lib"
 sonames="liblua.so.5.1.5 liblua5.1.so.5.1 liblua5.1.so.0 liblua.so.5.1 liblua-5.1.so liblua5.1.so"
 

thirdparty/fetch-thirdparty-deps.sh

@@ -1,5 +1,10 @@
 #!/bin/sh
 
+####
+# This file must stay /bin/sh and POSIX compliant for BSD portability.
+# Copy-paste the entire script into http://shellcheck.net to check.
+####
+
 # Die on any error for Travis CI to automatically retry:
 set -e
 

thirdparty/noget.sh

@@ -1,6 +1,11 @@
 #!/bin/sh
 # fallback without dependency resolution if nuget is not present on the system
 
+####
+# This file must stay /bin/sh and POSIX compliant for BSD portability.
+# Copy-paste the entire script into http://shellcheck.net to check.
+####
+
 archive="$1"
 version="$2"
 curl -o "$archive.zip" -Ls https://nuget.org/api/v2/package/"$archive"/"$version"