allow additional search paths (#25)

For certain build systems (e.g. nix, guix, spack) we need to tell
libwhich about non-standard search paths. Currently the only way of
doing so is by setting `LD_LIBRARY_PATH`, which difficult to scope just
to libwhich execution, and as a result can influence the runtime
behavior of other executables.

This patch allows you to set `LIBWHICH_LIBRARY_PATH=x:y:z`, which
libwhich uses to set `LD_LIBRARY_PATH` before it reruns itself, so
`dlopen(...)` can locate libraries in these search paths.

Author: haampie

libwhich.c

@@ -133,8 +133,16 @@ int mainCRTStartup(void) // actually is WINAPI without mangling (but ABI is the
 #else
 #include <dlfcn.h>
 #include <stdio.h>
+#include <unistd.h>
 typedef char WCHAR;
 #define T(str) str
+
+#if defined(__APPLE__)
+#define LIBWHICH_LD_LIBRARY_PATH "DYLD_LIBRARY_PATH"
+#else
+#define LIBWHICH_LD_LIBRARY_PATH "LD_LIBRARY_PATH"
+#endif
+
 #endif
 
 #ifndef RTLD_LAZY
@@ -336,6 +344,19 @@ int main(int argc, STR *argv)
         fputs(T(")\n"), stdout);
         return 1;
     }
+#ifndef _WIN32
+    // If LIBWHICH_LIBRARY_PATH is set, re-run the program with LD_LIBRARY_PATH
+    // or DYLD_LIBRARY_PATH set instead, so dlopen picks up these library paths.
+    const char *libwhich_path = getenv("LIBWHICH_LIBRARY_PATH");
+    if (libwhich_path) {
+        setenv(LIBWHICH_LD_LIBRARY_PATH, libwhich_path, 1);
+        unsetenv("LIBWHICH_LIBRARY_PATH");
+        if (execvp(argv[0], (char **)argv) == -1) {
+            perror("libwhich: execvp");
+            return 1;
+        }
+    }
+#endif
     struct vector_t before = dllist();
     void *lib = dlopen(libname, RTLD_LAZY);
     if (!lib) {