Zstd workaround for hylo-lang/llvm-build#8 (#34)

Work around hylo-lang/llvm-build#8

This finds libzstd using `pkg-config` instead of relying on the `RUNPATH` embedded in `llvm-config`, if `llvm-config` doesn't run without help.

Tools/make-pkgconfig.sh

@@ -1,5 +1,20 @@
 #!/usr/bin/env bash
 set -e
+set -o pipefail
+
+# Work around https://github.com/hylo-lang/llvm-build/issues/8
+#
+# We need to be resilient to no libzstd being found by pkg-config, as it is apparently not on linux.
+zstd_dash_L="$(pkg-config --silence-errors --libs-only-L libzstd || true)"
+if ! (llvm-config > /dev/null 2>&1); then
+  if [[ "$OSTYPE" == "linux-gnu"* || "$OSTYPE" == "cygwin" || "$OSTYPE" == "freebsd"* ]]; then
+    export LD_LIBRARY_PATH="${zstd_dash_L#-L}:$LD_LIBRARY_PATH"
+  elif [[ "$OSTYPE" == "darwin"* ]]; then
+    export DYLD_LIBRARY_PATH="${zstd_dash_L#-L}:$DYLD_LIBRARY_PATH"
+  elif [[ "$OSTYPE" == "msys" || "$OSTYPE" == "win32" ]]; then
+    export PATH="${zstd_dash_L#-L}:$PATH"
+  fi
+fi
 
 version=$(llvm-config --version)
 filename=$1
@@ -20,7 +35,7 @@ echo Name: LLVM > $filename
 echo Description: Low-level Virtual Machine compiler framework >> $filename
 echo Version: $(echo ${version} | sed 's/\([0-9.]\+\).*/\1/') >> $filename
 echo URL: http://www.llvm.org/ >> $filename
-echo Libs: -L$(llvm-config --libdir) $(pkg-config --libs-only-L libzstd) ${libs} $(llvm-config --system-libs --libs analysis bitwriter core native passes target) >> $filename
+echo Libs: -L$(llvm-config --libdir) ${zstd_dash_L} ${libs} $(llvm-config --system-libs --libs analysis bitwriter core native passes target) >> $filename
 echo Cflags: -I$(llvm-config --includedir) >> $filename
 
 echo "$filename written:"