sys: permit disabling static linking

This commit makes it so that if PCRE2_SYS_STATIC is explicitly set to 0,
then no static linking will occur. If PCRE2_SYS_STATIC isn't set, then
static linking will continue to be automatically enabled on MUSL
targets.

Fixes #7

pcre2-sys/README.md

@@ -44,7 +44,9 @@ PCRE2 system library. If that isn't available or if static linking is desired,
 then PCRE2 is built from source and statically linked.
 
 Static linking will automatically happen for MUSL targets, but can be forced by
-setting the `PCRE2_SYS_STATIC` environment variable to `1`.
+setting the `PCRE2_SYS_STATIC` environment variable to `1`. Similarly, if
+`PCRE2_SYS_STATIC` is set to `0`, then static linking will be forcefully
+disabled, even for MUSL targets.
 
 Currently, this crate only supports `libpcre-8` where
 `PCRE2_CODE_UNIT_WIDTH=8`.

pcre2-sys/build.rs

@@ -63,9 +63,7 @@ fn main() {
     let out = PathBuf::from(env::var_os("OUT_DIR").unwrap());
 
     // Don't link to a system library if we want a static build.
-    let want_static =
-        env::var("PCRE2_SYS_STATIC").unwrap_or(String::new()) == "1"
-        || target.contains("musl");
+    let want_static = pcre2_sys_static().unwrap_or(target.contains("musl"));
     if !want_static && pkg_config::probe_library("libpcre2-8").is_ok() {
         return;
     }
@@ -138,3 +136,18 @@ fn has_git() -> bool {
         .map(|s| s.success())
         .unwrap_or(false)
 }
+
+fn pcre2_sys_static() -> Option<bool> {
+    match env::var("PCRE2_SYS_STATIC") {
+        Err(_) => None,
+        Ok(s) => {
+            if s == "1" {
+                Some(true)
+            } else if s == "0" {
+                Some(false)
+            } else {
+                None
+            }
+        }
+    }
+}