Modify build_detect_platform to run fbcode.*.* irrespective of $PATH

Summary: This caused crash when the jenkins machine changed because the PATH variable is different.

Test Plan: make

Reviewers: dhruba, sheki

Reviewed By: sheki

CC: leveldb

Differential Revision: https://reviews.facebook.net/D10803

build_detect_platform

@@ -31,9 +31,9 @@ fi
 # Default to fbcode gcc on internal fb machines
 if [ -d /mnt/gvfs/third-party -a -z "$CXX" ]; then
     if [ -z "$USE_CLANG" ]; then
-        source fbcode.gcc471.sh
+        source ./fbcode.gcc471.sh
     else
-        source fbcode.clang31.sh
+        source ./fbcode.clang31.sh
     fi
 fi