cmake: fix tesseract detection

[alalek]

resolves #1378

tesseract 4.x headers require C++11 (can't be used by OpenCV 3.4 without C++11)

Workaround:

• (OpenCV 3.4.x) build in C++11 mode: cmake -DENABLE_CXX11=ON ...

Debug:

• cmake -DOPENCV_DEBUG_TESSERACT_BUILD=1 ...

Error messages:

In file included from /build/precommit_macosx/3.4/opencv_contrib/modules/text/src/ocr_tesseract.cpp:55:
In file included from /usr/local/Cellar/tesseract/4.1.0/include/tesseract/baseapi.h:26:
In file included from /usr/local/Cellar/tesseract/4.1.0/include/tesseract/apitypes.h:23:
/usr/local/Cellar/tesseract/4.1.0/include/tesseract/publictypes.h:33:1: error: unknown type name 'constexpr'
constexpr int kPointsPerInch = 72;
^
/usr/local/Cellar/tesseract/4.1.0/include/tesseract/publictypes.h:33:11: error: expected unqualified-id
constexpr int kPointsPerInch = 72;

opencv=build_warnings_xcode_10_3

[alalek]

👍