Added a compile time flag to compile SQLite with builtin math functions (#938)

* Added a compile time flag to compile SQLite with builtin math funcions

* Added a test, improved the comments in cargo config file

Author: francesco-cattoglio

.cargo/config.toml

@@ -0,0 +1,7 @@
+[env]
+# To use built-in math functions, this compile time flag must be set
+# See https://www.sqlite.org/draft/lang_mathfunc.html as a reference
+# According to Cargo docs this will not overwrite any env var that was already
+# set by the user, and this is a good thing. If the user already set some
+# LIBSQLITE3_FLAGS, he probably knows what he is doing.
+LIBSQLITE3_FLAGS = "-DSQLITE_ENABLE_MATH_FUNCTIONS"

tests/sql_test_files/it_works_sqrt.sql

@@ -0,0 +1,7 @@
+set number_three = sqrt(9.0);
+
+select 'text' as component, 
+    case $number_three
+        when '3.0' then 'It works !'
+        else 'error: ' || coalesce($number_three, 'NULL')
+    end AS contents;