Check for FDB C library early on, add /usr/lib64 to Linux search path

Getting a warning at the end of the script is confusing as to the status
of the operation. Did it complete? Partially complete? What was done? It
also wastes time as the repo download comes first.

On Linux, the library may be in /usr/lib64 in many cases, so check for
the library in that path too.

bindings/go/fdb-go-install.sh

@@ -23,8 +23,32 @@ if [[ "${platform}" == "Darwin" ]] ; then
     FDBLIBDIR="${FDBLIBDIR:-/usr/local/lib}"
     libfdbc="libfdb_c.dylib"
 elif [[ "${platform}" == "Linux" ]] ; then
-    FDBLIBDIR="${FDBLIBDIR:-/usr/lib}"
     libfdbc="libfdb_c.so"
+    custom_libdir="${FDBLIBDIR:-}"
+    FDBLIBDIR=""
+
+    if [[ -z "${custom_libdir}" ]]; then
+	search_libdirs=( '/usr/lib' '/usr/lib64' )
+    else
+	search_libdirs=( "${custom_libdir}" )
+    fi
+
+    for libdir in "${search_libdirs[@]}" ; do
+        if [[ -e "${libdir}/${libfdbc}" ]]; then
+            FDBLIBDIR="${libdir}"
+            break
+        fi
+    done
+
+    if [[ -z "${FDBLIBDIR}" ]]; then
+        echo "The FoundationDB C library could not be found in any of:"
+        for libdir in "${search_libdirs[@]}" ; do
+            echo "   ${libdir}"
+        done
+        echo "Your installation may be incomplete, or you need to set a custom FDBLIBDIR."
+        let status="${status} + 1"
+    fi
+
 else
     echo "Unsupported platform ${platform}".
     echo "At the moment, only macOS and Linux are supported by this script."
@@ -277,13 +301,7 @@ else
             cgo_ldflags="-L${FDBLIBDIR}"
             fdb_go_path="${REMOTE}/${FDBREPO}/bindings/go/src"
 
-            if [[ ! -e "${FDBLIBDIR}/${libfdbc}" ]] ; then
-                # Just a warning. Don't fail script.
-                echo
-                echo "WARNING: The FoundationDB C library was not found within ${FDBLIBDIR}."
-                echo "Your installation may be incomplete."
-                echo
-            elif ! CGO_CPPFLAGS="${cgo_cppflags}" CGO_CFLAGS="${cgo_cflags}" CGO_LDFLAGS="${cgo_ldflags}" go install "${fdb_go_path}/fdb" "${fdb_go_path}/fdb/tuple" "${fdb_go_path}/fdb/subspace" "${fdb_go_path}/fdb/directory" ; then
+            if ! CGO_CPPFLAGS="${cgo_cppflags}" CGO_CFLAGS="${cgo_cflags}" CGO_LDFLAGS="${cgo_ldflags}" go install "${fdb_go_path}/fdb" "${fdb_go_path}/fdb/tuple" "${fdb_go_path}/fdb/subspace" "${fdb_go_path}/fdb/directory" ; then
                 let status="${status} + 1"
                 echo "Could not build FoundationDB go libraries."
             fi