feat: allow setting sqlite compiler flags in extconf

Closes #401

Author: flavorjones

INSTALLATION.md

@@ -73,6 +73,45 @@ user    0m23.361s
 sys     0m5.839s
 ```
 
+##### Controlling compilation flags for sqlite
+
+Upstream sqlite allows for the setting of some parameters at compile time. If you're an expert and would like to set these, you may do so at gem install time in two different ways ...
+
+**If you're installing the gem using `gem install`** then you can pass in these compile-time flags like this:
+
+``` sh
+gem install sqlite3 --platform=ruby -- \
+  --with-sqlite-cflags="-DSQLITE_DEFAULT_CACHE_SIZE=9999 -DSQLITE_DEFAULT_PAGE_SIZE=4444"
+```
+
+or the equivalent:
+
+``` sh
+CFLAGS="-DSQLITE_DEFAULT_CACHE_SIZE=9999 -DSQLITE_DEFAULT_PAGE_SIZE=4444" \
+  gem install sqlite3 --platform=ruby
+```
+
+**If you're installing the gem using `bundler`** then you should first pin the gem to the "ruby" platform gem, so that you are compiling from source:
+
+``` ruby
+# Gemfile
+gem "sqlite3", force_ruby_platform: true # requires bundler >= 2.3.18
+```
+
+and then set up a bundler config parameter for `build.sqlite3`:
+
+``` sh
+bundle config set build.sqlite3 \
+  "--with-sqlite-cflags='-DSQLITE_DEFAULT_CACHE_SIZE=9999 -DSQLITE_DEFAULT_PAGE_SIZE=4444'"
+```
+
+NOTE the use of single quotes within the double-quoted string to ensure the space between compiler flags is interpreted correctly. The contents of your `.bundle/config` file should look like:
+
+``` yaml
+---
+BUNDLE_BUILD__SQLITE3: "--with-sqlite-cflags='-DSQLITE_DEFAULT_CACHE_SIZE=9999 -DSQLITE_DEFAULT_PAGE_SIZE=4444'"
+```
+
 
 #### System libsqlite3
 

ext/sqlite3/extconf.rb

@@ -51,12 +51,13 @@ def configure_packaged_libraries
         minimal_recipe.tap do |recipe|
           recipe.configure_options += ["--enable-shared=no", "--enable-static=yes"]
           ENV.to_h.tap do |env|
-            additional_cflags = [
+            user_cflags = with_config("sqlite-cflags")
+            more_cflags = [
               "-fPIC", # needed for linking the static library into a shared library
               "-O2", # see https://github.com/sparklemotion/sqlite3-ruby/issues/335 for some benchmarks
               "-fvisibility=hidden", # see https://github.com/rake-compiler/rake-compiler-dock/issues/87
             ]
-            env["CFLAGS"] = [env["CFLAGS"], additional_cflags].flatten.join(" ")
+            env["CFLAGS"] = [user_cflags, env["CFLAGS"], more_cflags].flatten.join(" ")
             recipe.configure_options += env.select { |k,v| ENV_ALLOWLIST.include?(k) }
                                            .map { |key, value| "#{key}=#{value.strip}" }
           end
@@ -234,17 +235,23 @@ def print_help
 
             Flags only used when building and using the packaged libraries:
 
+                --with-sqlite-cflags=CFLAGS
+                    Explicitly pass compiler flags to the sqlite library build. These flags will
+                    appear on the commandline before any flags set in the CFLAGS environment
+                    variable. This is useful for setting compilation options in your project's
+                    bundler config. See INSTALLATION.md for more information.
+
                 --enable-cross-build
                     Enable cross-build mode. (You probably do not want to set this manually.)
 
 
-            Environment variables used for compiling the C extension:
+            Environment variables used for compiling the gem's C extension:
 
                 CC
                     Use this path to invoke the compiler instead of `RbConfig::CONFIG['CC']`
 
 
-            Environment variables passed through to the compilation of packaged libraries:
+            Environment variables passed through to the compilation of sqlite:
 
                 CC
                 CPPFLAGS