component/tesseract: declare a minimal gcc version

With gcc 6.3.0 we see build failures:

    error: no matching function for call to 'tesseract::STRING::STRING()'

According to https://github.com/tesseract-ocr/tesseract/issues/3210#issuecomment-774599019
minimal gcc version is version 7.

This probably started to break since d6dd75df (component/ghostscript:
version up 9.54.0, 2021-05-25) and 5f616dd7 (component/tesseract: version
up 4.1.1, 2021-05-20).

This gcc is needed for both tesseract and ghostscript, because ghostscript
build again its embedded copy of tesseract.

component/tesseract/buildout.cfg

@@ -14,6 +14,10 @@ extends =
 parts =
   tesseract
 
+[gcc]
+# tesseract uses some C++ 17 features which needs gcc >= 7
+min_version = 7
+
 [tesseract]
 recipe = slapos.recipe.cmmi
 shared = true