cAddSearchDir + cSearchPath stopped working [regression]
Hello,

I am building nimterop from source after a break of about 3 months.

Below code (`svdpi.nim`) used to work:

```nim
import nimterop/cimport
import os

const
  xlmIncludePath = getEnv("XCELIUM_ROOT") / ".." / "include"
static:
  doAssert fileExists(xlmIncludePath / "svdpi.h")
  doAssert fileExists(xlmIncludePath / "svdpi_compatibility.h")

cDefine("DPI_COMPATIBILITY_VERSION_1800v2012")

# Below manual definition of s_vpi_vecval is a workaround for
# https://github.com/genotrance/nimterop/issues/47.
type
  s_vpi_vecval* {.importc: "s_vpi_vecval", header: xlmIncludePath / "svdpi.h".} = object
    aval*: uint32 # we need to export the object elements too!
    bval*: uint32

cAddSearchDir(xlmIncludePath)
cImport(cSearchPath("svdpi.h"))
```

This still builds, but when I do `import svpi`, I get this error:

```
Hint: time_t [Processing]
stack trace: (most recent call last)
../../../../../.nimble/pkgs/svdpi-0.1.0/svdpi.nim(23, 20) svdpi
../../../../../.nimble/pkgs/nimterop-0.1.0/nimterop/cimport.nim(271, 11) cSearchPath
../../../../../usr_local/apps/6/nim/devel/lib/system/assertions.nim(27, 20) failedAssertImpl
../../../../../usr_local/apps/6/nim/devel/lib/system/assertions.nim(20, 11) raiseAssert
../../../../../usr_local/apps/6/nim/devel/lib/system/fatal.nim(37, 5) sysFatal
/home/kmodi/usr_local/apps/6/nim/devel/lib/system/fatal.nim(37, 5) Error: unhandled exception: /home/kmodi/.nimble/pkgs/nimterop-0.1.0/nimterop/cimport.nim(271, 14) `found` File or directory not found: svdpi.h gStateCT.searchDirs: @[]
```

The workaround is to not use cAddSearchDir + cSearchPath .. so after this diff, `import svdpi` works fine:

```diff
diff --git a/src/svdpi.nim b/src/svdpi.nim
index 5662093..82b86d1 100644
--- a/src/svdpi.nim
+++ b/src/svdpi.nim
@@ -19,5 +19,4 @@ type
     aval*: uint32 # we need to export the object elements too!
     bval*: uint32
 
-cAddSearchDir(xlmIncludePath)
-cImport(cSearchPath("svdpi.h"))
+cImport(xlmIncludePath / "svdpi.h")
```

---

## Nim version

```
Nim Compiler Version 0.19.9 [Linux: amd64]
Compiled at 2019-04-03
Copyright (c) 2006-2019 by Andreas Rumpf

git hash: dc2986789b703300ac0680a10004f77453b146c3
active boot switches: -d:release
```