build: raise minimum build requirements

 * Hard-depend on gcc >= 7.4 and clang >= 5.0, which have
   almost complete C++17 support
 * Require Xcode 10 on macOS but recommend 11.3 or later
 * Boost >= 1.65.1 is now a hard requirement

Change-Id: I76eb915b226e8f5dd5c5fdbde9b23befc2a37c90
diff --git a/INSTALL.md b/INSTALL.md
index 8e0cd24..fdd94e5 100644
--- a/INSTALL.md
+++ b/INSTALL.md
@@ -4,26 +4,26 @@
 
 ## Prerequisites
 
--  Install the [ndn-cxx](https://named-data.net/doc/ndn-cxx/current/) library and its prerequisites.
-   Please see [Getting Started with ndn-cxx](https://named-data.net/doc/ndn-cxx/current/INSTALL.html)
-   for how to install ndn-cxx.
-   Note: If you have installed ndn-cxx from a binary package, please make sure development headers
-   are installed (e.g., if using Ubuntu PPA, the `libndn-cxx-dev` package is needed).
+- Install the [ndn-cxx](https://named-data.net/doc/ndn-cxx/current/) library and its prerequisites.
+  Please see [Getting Started with ndn-cxx](https://named-data.net/doc/ndn-cxx/current/INSTALL.html)
+  for instructions.
+  All operating systems and compilers supported by ndn-cxx are also supported by ndn-tools.
 
-   Any operating system and compiler supported by ndn-cxx is supported by ndn-tools.
+  *Note*: If you have installed ndn-cxx from a binary package, please make sure the development
+  headers are installed (e.g., if using Ubuntu PPA, the `libndn-cxx-dev` package is needed).
 
--  `libpcap`
+- `libpcap`
 
-    Comes with the base system on macOS.
+  Comes with the base system on macOS.
 
-    On Ubuntu:
+  On Ubuntu:
 
-        sudo apt install libpcap-dev
+      sudo apt install libpcap-dev
 
-    On CentOS and Fedora:
+  On CentOS and Fedora:
 
-        sudo dnf config-manager --enable powertools  # CentOS only
-        sudo dnf install libpcap-devel
+      sudo dnf config-manager --enable powertools  # CentOS only
+      sudo dnf install libpcap-devel
 
 ## Build Steps
 
@@ -34,6 +34,6 @@
     ./waf
     sudo ./waf install
 
-To uninstall ndn-tools:
+To uninstall:
 
     sudo ./waf uninstall